Lymphocyte

A lymphocyte is a type of white blood cell that forms the core of the immune system. Lymphocytes are agranulocytes, cells with a relatively large nucleus and only a thin rim of cytoplasm, and they make up a substantial share of circulating white blood cells, often quoted as roughly 20–30%. Although they look alike under a light microscope, they are grouped into three main functional classes: B cells, which produce antibodies; T cells, which recognize and act against antigens presented on cell surfaces; and natural killer (NK) cells, which belong to the innate arm of immunity. All three arise from a common lymphoid progenitor within the hematopoietic stem cell lineage.

Lymphocytes matter because they link recognition to response. Their receptors are generated by somatic gene rearrangement, which gives the adaptive immune system the capacity to target an almost unlimited range of pathogens, while memory cells allow faster and stronger responses on re-exposure. Disorders of lymphocyte number or function underlie immunodeficiency, autoimmunity, and several cancers, and lymphocytes are now direct therapeutic targets in treatments such as checkpoint inhibitors and engineered T cell therapies. This entry covers how lymphocytes are classified and produced, how each major type works, how they circulate and are activated, how they are measured clinically, what happens when they fail, and how their biology was discovered.

Definition and classification

Lymphocytes are a group of white blood cells classified as agranulocytes, meaning their cytoplasm lacks the coarse granules seen in neutrophils, eosinophils, and basophils. In light microscopy they appear as small round cells with a relatively large, often round nucleus and a thin rim of cytoplasm. Estimates of their share of circulating leukocytes vary by source and population: one overview gives 20–30%, while another puts the figure at about 30%. In most adult blood counts they are the second most common leukocyte after neutrophils.

Lymphocytes look much alike under a standard stain, and this uniformity is one reason the classes are defined by function rather than appearance. A small resting B cell and a small resting T cell are difficult to tell apart by eye, and many lymphocytes are identified only by their surface molecules or by what they secrete or do when stimulated. Activated lymphocytes are somewhat larger, with more cytoplasm and a more developed endoplasmic reticulum and Golgi apparatus, which is why morphology alone gives limited information about lineage.

Three principal classes are recognized: B cells, T cells, and natural killer (NK) cells. B cells mediate humoral immunity, chiefly by producing antibodies. T cells mediate cell-mediated immunity, either by directly killing infected or malignant cells or by coordinating other immune cells. NK cells destroy virus-infected and transformed cells without prior sensitization. The first two classes belong to the adaptive immune system, because their antigen receptors are generated by somatic gene rearrangement and each clone recognizes a particular antigen. NK cells belong to the innate immune system, because they use germline-encoded receptors and do not require antigen-specific clonal expansion for their first response. NK cells are also grouped with the broader family of innate lymphoid cells, and they are sometimes described as large granular lymphocytes; in human blood they make up roughly 5–20% of circulating lymphocytes.

Beyond these three classes, each lineage is subdivided. Subsets are commonly defined by surface molecules, especially members of the cluster of differentiation (CD) family, and by effector function. T cells divide into helper, cytotoxic, regulatory, and memory populations, and B cells into naive, memory, plasmablast, and plasma cell stages. The categories overlap in practice. Plasma cells are B-lineage cells that have largely lost the surface markers used to identify naive B cells. Some T cells that express an innate-like receptor repertoire behave more like innate cells, and some cells carry markers from more than one lineage. For this reason, classification schemes are working frameworks: they are useful for clinical and research purposes, but the boundaries between subsets are often gradual rather than sharp.

Development and lineage

From stem cell to lymphoid progenitor

All lymphocytes derive from hematopoietic stem cells (HSCs) in the bone-marrow. The lymphoid branch of hematopoiesis has traditionally been traced through the common lymphoid progenitor (CLP), a cell that comes from a hematopoietic stem cell and is the point where the cell line is no longer heading toward red blood cells, platelets, or most myeloid cells. In the classical model, CLPs produce T, B and NK cells alike and cannot give rise to myeloid cells. However, studies on early thymic progenitors suggested that T cell development might not follow this classical pathway. Later work showed that lymphoid-primed multipotent progenitors (LMPPs) in the marrow are potent T-lineage progenitors and can reach the thymus through the blood without passing through the CLP stage, so the CLP is best viewed as one of several routes rather than an obligatory step.

Fetal development follows a different geography. Lymphoid cells are first produced in the yolk sac, then in the fetal liver, and finally in the bone marrow, which becomes the main site after birth.

Maturation sites

B lymphocytes and T lymphocytes mature in separate central organs. The bulk of lymphocyte development in mammals takes place in the bone marrow (and, in the fetus, the liver) for B cells, and in the thymus for T cells. Development in these antigen-independent compartments is followed by antigen-dependent maturation in the spleen, lymph nodes, tonsils and mucosa-associated tissues such as Peyer's patches.

B cells complete their early stages in the marrow, where their progress is tracked by the state of immunoglobulin gene rearrangement and by the surface markers each stage expresses. T cell precursors instead leave the marrow and enter the thymus. Their entry depends on adhesion and chemokine signals; in mice, the CLP-2 subset migrates to the thymus more efficiently than other marrow progenitors, which correlates with higher expression of the chemokine receptor CCR9. Inside the thymic cortex, the precursors pass through the double-negative, double-positive and single-positive stages under the influence of cytokines produced by thymic epithelial cells. The name "T cell" reflects this thymic dependence.

NK cells share the lymphoid origin but do not follow the same maturation path as B or T cells. After childhood, thymic output of new T cells declines, and the peripheral T cell pool is maintained largely by division of mature cells outside the central organs.

V(D)J recombination

Antigen receptor diversity is generated by V(D)J recombination, a somatic DNA rearrangement carried out in developing B and T cells. Variable (V), diversity (D) and joining (J) gene segments are selected and joined so that one segment of each type forms a single exon encoding the antigen-binding region. The reaction is initiated by the RAG1 and RAG2 proteins, which cut DNA next to recombination signal sequences that flank each segment. The broken ends are then repaired by the nonhomologous end-joining pathway, and the joined segments are generated in a cut-and-paste manner. D segments occur in the heavy chains of immunoglobulin and in the β chain of the T cell receptor, but not in light chains.

Diversity arises from three sources: the combinatorial choice of segments, imprecise trimming and addition of nucleotides at the junctions, and the pairing of different heavy and light chains (or α and β chains). The junction region encoding the most variable part of the receptor, known as CDR3, therefore carries the greatest sequence variety. The same process has costs. Imprecise joining frequently yields out-of-frame sequences that cannot produce protein, and some rearrangements generate receptors that bind self-molecules, which is why selection is necessary.

Selection checkpoints

Developing lymphocytes pass through quality-control checkpoints that favor receptors which are functional and not strongly self-reactive. In developing B cells, rearrangements that produce a functional receptor signal survival and further differentiation, whereas cells whose receptors are highly self-reactive are removed through clonal-deletion, or alternatively are altered by further rearrangement (receptor editing) or rendered anergic.

T cell selection is the best-studied example. In the thymus, the first checkpoint is β-selection among double-negative cells, which retains only cells with a functional TCRβ chain. Surviving cells become CD4⁺CD8⁺ double-positive thymocytes and undergo positive selection, which requires their receptors to recognize self-peptides presented on MHC molecules. Cells that fail this test die by neglect. Negative selection then removes thymocytes that bind self-peptide–MHC complexes too strongly. A proposed explanation, the differential avidity hypothesis, holds that the same ligand delivers a weak signal that rescues cells in positive selection and a stronger signal that triggers death in negative selection. Experiments with tetramer labeling later showed that negative selection can begin before positive selection is evident, so the two processes may proceed independently. Thymic B cells and other antigen-presenting cells contribute to negative selection, and some moderate-affinity thymocytes escape deletion and become peripheral T cells capable of responding to self. These checkpoints together constitute central-immune-tolerance; when they malfunction, the result can be autoimmunity.

B lymphocytes

B cell receptor and forms of immunoglobulin

The B cell receptor (BCR) is an antibody-like molecule anchored in the membrane, where disulfide bridges join two heavy-light chain pairs into one unit. A 3.3-angstrom cryo-electron microscopy structure of the human IgM-BCR in its resting state shows that the complex carries two heavy chains, two light chains, and the Igα/Igβ heterodimer. Signaling depends on the Igα and Igβ subunits, each of which has an intracellular immunoreceptor tyrosine-based activation motif (ITAM). Antigen binding causes receptors to cluster, which leads to ITAM phosphorylation.

Naive B cells display more than one isotype. Membrane IgM and IgD share the same antigen specificity, and IgD arises through alternative splicing of the same gene, so both are co-expressed. Mature naïve IgM+IgD+ B cells circulate through the follicles of secondary lymphoid tissues until they encounter antigen. Membrane and secreted immunoglobulin share the same antigen-binding region, and differing only at the C-terminal end by alternative mRNA processing is what separates them. The cytoplasmic tails differ markedly in length: the membrane IgM and IgD tails are only 3 amino acids long, whereas membrane IgG1–4 have a conserved tail of 29 amino acids.

Activation and plasma cell differentiation

When a polyvalent antigen cross-links several receptors, the cell proliferates and differentiates. A naive cell that meets its cognate antigen divides and becomes a plasma cell that secretes antibodies with the same specificity as its receptor. Secreted IgM forms a pentamer, which gives it efficient complement activation and strong clearance of multivalent antigens.

Responses proceed along two routes. Following activation, antigen-specific B cells are able to progress immediately into short-lived PCs without T-cell help. These cells exist only transiently in the spleen and lymph nodes, and do not undergo affinity maturation. The second route generates long-lived plasma cells of higher affinity than their short-lived counterparts, which can survive for many years and are generated within the germinal centres.

Germinal centers: class switching and somatic hypermutation

Germinal centers are the microanatomic setting in which antibody affinity is refined. Mutation of the variable region takes place there, and cells whose receptors lose affinity after mutation are removed by apoptosis, while those whose affinity rises migrate to the light zone for further selection. Somatic hypermutation is restricted to activated B cells. It primarily occurs in the germinal centers of the lymph nodes and spleen.

Class switching changes the constant region while leaving the antigen-binding region intact. The C_H region is the region that is replaced in class switching, while the variable regions are targeted in somatic hypermutation. A single enzyme drives both processes. Their independent roles in regulating B cell fate have been difficult to uncouple because a single enzyme, activation-induced cytidine deaminase, initiates both reactions. Cytokines from helper T cells help determine which isotype is chosen. Timing is also more nuanced than once assumed. One study found that CSR is initiated over the first few days in a primary response and prior to EF and GC commitment, ceasing soon after B cells become GC cells and SHM commences. Phylogenetic analysis of human germinal center receptors indicated that the vast majority of CSR events occurred prior to the onset of somatic hypermutation.

The randomness of mutation carries risk as well as benefit. The random nature of somatic hypermutation can introduce novel reactivities, which makes tolerance mechanisms in the germinal center important. Mutation can also shape the memory compartment directly: SHM reduced the longevity of memory B cells by creating polyreactive specificities that were selected against over time.

Memory B cells and long-lived humoral immunity

Humoral memory has two components. B cell memory is bipartite, consisting of long-lived antibody-secreting plasma cells (humoral memory) and long-lived memory B cells, able to react quickly to a recurrent antigenic challenge. Memory B cells do not secrete antibody at rest. Following appropriate stimulation, these cells proliferate and differentiate into antibody-secreting cells. Many of them carry exceptionally high affinity antigen-specific B cell receptors, and they serve as a critical source of the long-lived plasma cells that secrete protective serum antibodies.

The traditional account held that memory cells continually replenish the plasma cell pool. Conventional models suggest that long-term antibody responses are maintained by the continuous differentiation of memory B cells into antibody-secreting plasma cells. Later work challenged this view. In one mouse study, a substantial fraction of plasma cells can survive and continue to secrete antibody for extended periods of time (>1 year) in the absence of any detectable memory B cells. Human data point in the same direction. Serum antibodies are short-lived, with serum half-lives ranging from ∼1 week to 1 month, yet responses can persist for a lifetime. In patients treated with CD19-directed CAR T-cell therapy, researchers observed B-cell–independent longevity of some human PCs.

T lymphocytes

Antigen recognition and MHC restriction

T cells recognize antigen through the T cell receptor (TCR), but they do not see free antigen in the way antibodies do. Instead, the TCR reads short peptide fragments that are displayed on the surface of other cells by MHC molecules. Early work established that helper CD4+ T cells responded exclusively to antigens presented by class II MHC, while CD8 cytotoxic T cells recognized antigen in the context of class I MHC presentation. This pairing is called MHC restriction. The two classes also differ in the peptides they carry: CD8+ T cells recognize viral peptides of 8–11 amino acids in length bound to MHC class I molecules, whereas CD4+ T cells recognize peptides of 12 or more amino acids restricted by MHC class II. The CD4 and CD8 coreceptors help stabilize this interaction by binding MHC at sites outside the peptide-binding groove, with CD4 binding to the β2 domain of MHC class II, while the binding site of CD8 has been mapped to the α3 domain of MHC class I. Restriction is a strong rule rather than an absolute one. Rare HLA class II-restricted CD8+ T cell responses with antiviral properties have been described in a small group of people with chronic HIV infection.

CD4 and CD8 lineages

T cells pass through the thymus, where they first express both CD4 and CD8 and then commit to one lineage. During thymic selection, double-positive T cells undergo a maturation process where they are selected as either CD8+ T cells or CD4+ T cells depending on TCR binding to peptide complexed with MHC class I or class II, respectively. The outcome is that CD4 cells become helper cells and CD8 cells become cytotoxic cells. Both lineages are generated in the thymus and carry the αβ T cell receptor.

Helper T cell subsets

CD4 helper cells are divided into subsets according to the cytokines they produce and the master transcription factors that drive them. Th1 (IFN-γ and T-bet), Th2 (IL-4/IL-5/IL-13 and GATA3), Th17 (IL-17/IL-22 and RORγt), Tfh (IL-21 and Bcl6) and Treg (IL-10/TGF-β/IL-35 and Foxp3) represent the five commonly recognized groups. Their differentiation is guided by the cytokine environment present during activation, since particular combinations of cytokines induce expression of master transcription factors such as T-bet, GATA-3, RORγt, Bcl6, or Foxp3. Each subset supports a different kind of immune response. Th1 cells secrete interferon gamma and create a milieu in which key cytotoxic effectors—macrophages, natural killer cells, and cytotoxic CD8+ T lymphocytes—are activated. Th2 cells help antigen-primed B lymphocytes differentiate into plasma cells and secrete antibodies. Th17 cells, through IL-17, recruit neutrophils and monocytes to infected sites. Tfh cells help B cells produce antibodies, and their roles in germinal centers are described elsewhere. Some additional subsets, such as Th9 and Tr1, have been proposed, but there are still controversial issues in defining them as unique lineage subsets, partly because no clear master regulator has been identified for each.

Cytotoxic T cells

CD8 cytotoxic T lymphocytes are a central part of defense against intracellular pathogens and tumors. When the TCR engages its MHC class I ligand, the cell releases the contents of its cytotoxic granules. Engagement triggers the synthesis of perforin and granzymes, which are stored within the cytosol. Perforin makes pores in the target membrane, allowing granzymes to enter and trigger death of the target cell. Some CD4 cells also kill. These cells are characterized by their ability to secrete granzyme B and perforin and to kill the target cells in an MHC class II-restricted fashion. Most of these CD4 cytotoxic cells derive from Th1-like cells.

Regulatory T cells

Regulatory T cells (Tregs) are the main subset that suppresses other lymphocytes. In humans, roughly 5-10% of the CD4+ T cell population in peripheral blood are comprised of naturally arising Tregs characterized by constitutive expression of CD25 and Forkhead Box 3 (FOXP3). FOXP3 is the defining transcription factor of this lineage. Foxp3 is widely recognized as the lineage determinant of CD4 Treg cells and is not expressed meaningfully by non-T cell types. Its importance is clear from human disease: mutations in FOXP3 were discovered to be associated with an autoimmune lymphoproliferative illness in humans, now known as IPEX syndrome. Depletion of these cells in animals also causes severe autoimmunity, allergy, and immunopathology.

Tregs suppress through several routes, including inhibiting co-stimulatory signals via CD80/CD86, competing for IL-2 (IL-2 sink), and secreting inhibitory cytokines such as IL-10, IL-35, and TGF-β. Their suppressive function also depends on maintaining high expression of CTLA-4. FOXP3 is not a perfect marker, however. It is transiently upregulated in Teff cells upon TCR-activation without endowing them with suppressive function, and human FOXP3-positive cells are functionally heterogeneous, whereby a sizeable proportion of FOXP3+ cells in healthy individuals have a diminished capacity to suppress responder cells. Tregs also have a dark side in cancer, where they suppress the function of various immune cells including CD4+ T helper cells, CD8+ cytotoxic T cells, NK cells, and NK T cells.

Natural killer cells

Natural killer (NK) cells were first identified in 1975 and were classified as lymphocytes on the basis of their morphology, their expression of lymphoid markers, and their origin from the common lymphoid progenitor in bone marrow. Because they lack antigen-specific surface receptors, they are generally counted among the innate rather than the adaptive arm of defense. Their lymphoid identity and their innate function are therefore two separate facts that together define the cell.

NK cells arise from a progenitor downstream of the common lymphoid progenitor, which also gives rise to T and B lymphocytes. They mature mainly in the bone marrow, although maturation also occurs in lymph nodes, the spleen, tonsils, and the thymus before cells enter the circulation. Unlike T cells, NK cells do not undergo RAG-dependent receptor rearrangement, which is why they are grouped with the innate lymphoid cells and are described as large granular lymphocytes. Human NK cells usually express CD16 and CD56, although a CD16-negative subset exists.

The killing machinery is stored in cytoplasmic granules. These contain perforin and a family of serine proteases called granzymes. When the granules are released onto a target, perforin inserts pores into the target membrane, allowing granzymes to enter and trigger apoptosis. Granzyme B is especially important for rapid induction of target cell death. A second route uses death-receptor ligands such as Fas ligand and TRAIL, which is slower than granule release but works through a separate pathway.

NK cell activity is governed by a balance of signals rather than by recognition of a specific antigen. Inhibitory receptors, namely killer cell immunoglobulin-like receptors (KIR) in humans, Ly49 molecules in mice, and CD94/NKG2A in both species, recognize MHC class I molecules on the surface of other cells. While a target keeps its MHC class I molecules, these receptors suppress NK activation. Cells that have lost MHC class I expression, as many tumor cells and some virus-infected cells do, escape this suppression and become targets. This "missing-self" principle was proposed in the 1980s and later confirmed as an inhibitory-receptor mechanism rather than a masking mechanism. Activating receptors, such as NKp30 and NKG2D, work in parallel. Their engagement triggers the secretion of lytic granules at the immune synapse. Activating receptors can also override inhibitory signals, which means that missing-self recognition is not the only route to killing.

NK cells must be "educated" through interactions between inhibitory receptors and self-MHC class I to acquire full functional competence; this process is often called licensing. Cells that never receive this signal can be hyporesponsive, and such NK cells can be found in human peripheral blood, where they fail to respond well even to MHC class I-deficient targets. This calibration helps explain how NK cells remain tolerant of healthy tissue while retaining the capacity to act against abnormal cells.

Beyond cytotoxicity, NK cells are major early producers of cytokines. They are among the first sources of interferon-γ during many infections, and this cytokine shapes subsequent responses. NK-derived interferon-γ can influence dendritic cells and naive T cells in lymph nodes, and it can enhance antigen presentation to T cells. In an experimental study of human peripheral blood mononuclear cells, interferon-γ production came specifically from CD16-positive cells and not from T cells.

NK cells belong to the innate lymphoid cell family, whose first discovered member they were. Their defining feature in this classification is that they are non-T, non-B lymphoid cells that can produce interferon-γ rapidly after stimulation. The contrast with T cells is structural: T cells assemble their antigen receptors through V(D)J recombination and so achieve antigen specificity, whereas NK cells carry no rearranged antigen receptor and rely on the germline-encoded receptor repertoire described above. Both lineages depend on the transcription factor T-bet, and ILC1 development in the mouse liver depends on interferon-γ acting on local progenitors, a process that does not drive NK cell development.

Circulation and lymphoid tissues

Recirculation between blood, lymph, and tissues

Mature lymphocytes do not stay in one place. Naive cells move from the blood into lymph nodes and other secondary lymphoid organs, then return to the circulation while they search for their specific antigen. The majority of T cells recirculate by crossing the lymph node high endothelial venules, but they can also reach other sites via afferent lymphatics, which drain, for example, inflammatory lesions or peripheral vascular beds. Once a naive lymphocyte is activated, its surface profile changes. The activated effector cells leave lymphoid tissues through the sphingosine 1-phosphate (S1P) receptor pathway and return to the blood. Effector lymphocytes lower their expression of CCR7 and L-selectin, but they raise the ligands for P-selectin and E-selectin, along with chemokine receptors and integrins, which allow them to pass through endothelium at sites of infection or inflammation. This shift explains why effector and memory cells are distributed differently from naive cells, which are concentrated in lymphoid organs.

Homing: adhesion molecules and chemokine signals

Entry into lymph nodes depends on a multistep adhesion cascade. Naive lymphocytes circulating in the blood enter lymph nodes through high endothelial venules (HEVs), which are specialized blood vessels lined by plump endothelial cells. The main adhesion molecule is L-selectin. HEVs express high levels of sulphated sialomucins decorated with 6-sulpho sialyl Lewis X, which are ligands for the lymphocyte homing receptor L-selectin. The cell first rolls along the vessel wall, then firms its attachment, crawls over the luminal surface, and finally passes through the endothelium. Rolling is initiated by L-selectin interactions with HEV sialomucins. Then the lymphocytes crawl on the HEV luminal surface. The cells rapidly transmigrate across the HEV endothelium via exit ramps. Integrin activation is required between rolling and transmigration. Chemokines provide the directional signal. Naive lymphocytes express the chemokine receptor CCR7, which helps them move from the blood into lymphoid tissues in response to the chemokines CCL19 and CCL21 released there. Inside the node, B cells then migrate toward follicles, and T cells toward the paracortex.

Secondary lymphoid organs as sites of antigen encounter

Secondary lymphoid organs are where naive cells meet antigen and begin an adaptive response. The secondary lymphoid organs are the sites of lymphocyte activation by antigens. Lymph nodes are the most densely packed: the highest lymphocyte concentration is found in the lymph nodes. Antigen-bearing dendritic cells arrive through afferent lymph, so nodes act as a local environment for antigen drainage and interaction with presenting populations to engage adaptive lymphocytes.

The spleen works differently because it filters blood rather than lymph. In the white pulp, lymphoid follicles house large numbers of B lymphocytes, and the periarteriolar lymphatic sheath contains T lymphocytes. The marginal zone is the region between the red pulp and white pulp, which sequesters particulate antigens from the circulation and presents these antigens to lymphocytes in the white pulp.

Mucosal lymphoid tissue covers the body's surfaces that face the outside environment. Mucosa-associated lymphoid tissue (MALT) is composed of cellular aggregates in the lamina propria of the digestive tract lining and respiratory tract. Its named components include the Peyer patches and tonsils. Lymphocytes primed in these inducer sites can later travel to effector sites in other mucosal tissues, which is the basis of the gut-to-airway immune connection.

The lymphatic system as a conduit

The lymphatic system provides the transport network linking tissues to these organs. It includes lymphatic vessels that carry a fluid called lymph, along with organs where immune cells develop and organs and tissues where immune cells collect ready to fight infection. Lymph carries both cells and material. Lymph contains waste products and cellular debris, together with bacteria and proteins. Lymph drains from the tissues, passes through the lymphatic vessels and lymph nodes, and enters the venous system through the right lymphatic duct and the thoracic duct. As these vessels pass through numerous lymph nodes, the nodes filter out unwanted materials such as bacteria and damaged cells. In this way, lymphatic flow delivers antigens and antigen-presenting cells to the nodes, while efferent lymph carries activated lymphocytes out to peripheral sites.

Activation, clonal expansion, and memory

Two-signal activation

Recognition of a target antigen is not enough, by itself, to switch on a naive T cell. The current model holds that naive T cells need two distinct signals. The first is supplied when the T-cell receptor meets a peptide displayed by MHC on an antigen-presenting cell, and it is this input that makes the response antigen-specific. The second, costimulatory input comes from B7 molecules on the antigen-presenting cell engaging CD28 on the T cell. CD28 on the T cell interacting with B7-1 (CD80) or B7-2 (CD86) on the antigen-presenting cell at least partly fulfills the requirement for this second signal.

The necessity of the second input was established experimentally. Stimulating T cells with anti-T-cell receptor antibodies, or with mitogenic lectins in the absence of viable antigen-presenting cells, produced neither clonal expansion nor interleukin-2 (IL-2) output. When receptor engagement arrives without the second input, the cell is functionally inactivated and enters a state of tolerance. Once both inputs arrive, CD28 signaling adds to the response: cytokine expression is markedly enhanced after CD28 costimulation, through effects on both transcription and mRNA stabilization, and CD28 also governs survival by inducing the anti-apoptotic protein Bcl-xL.

Costimulation is also regulated from the other side. CTLA-4 is induced after activation and competes for the same ligands. CTLA4, another receptor for B7 that is upregulated after T cell activation, can deliver an inhibitory signal that downregulates proliferation. Its binding affinity for B7 is reported at approximately 20-fold higher than that of CD28, which tilts the balance toward restraint. The same logic underlies the CTLA-4 and PD-1 inhibitors discussed elsewhere in this entry.

Clonal selection and expansion

The principle that organizes this process is clonal selection. Clonal selection holds that specific antigen receptors already exist on lymphocytes before any antigen is encountered, arising from random diversification during maturation. Antigen therefore does not create receptors; it picks out the cells that already carry a matching one. After antigen presentation, the selected lymphocytes undergo clonal expansion because they carry the needed receptor.

The scale of this expansion is large. In mice, clonal expansion during a primary infection can raise the precursor frequency of antigen-specific T cells roughly 1000-fold compared with naive animals. The expansion depends on the continued presence of antigen: antigen-specific T cells expand clonally during the first phase while antigen is present. CD8 cytotoxic cells proliferate faster and reach larger clonal sizes than CD4 helper cells, according to a review of T-cell differentiation for vaccine design.

Contraction and the formation of memory

After the pathogen is cleared, the response winds down. During the contraction phase, 90–95% of the T cells present at the peak of expansion die, leaving mainly memory T cells that persist long term. The expanded population is not uniform. It consists of a majority of short-lived terminal effectors, destined to die during contraction, and a much smaller population of memory precursors, destined for long-term maintenance. The size of the eventual memory pool is shaped by how many cells die, a process that may be influenced by interleukin-2 family cytokines, TNF-receptor family molecules, and effector molecules such as perforin and interferon-γ.

Memory B cells follow a comparable but distinct course. They are generated in the germinal center, where after somatic hypermutation and class switching, B cells differentiate into plasma cells and memory B cells. Memory B cells persist in a quiescent state in secondary lymphoid organs, tissues, and peripheral blood, poised for rapid reactivation when antigen is encountered again. Antibody maintenance is handled largely by a separate population: long-lived plasma cells reside in the bone marrow and provide long-lasting protection by continuously producing antibodies. The immunological memory that results is therefore carried by both cell types.

Primary versus secondary responses and vaccination

The first encounter with an antigen produces a slow response, and the second produces a faster one. In the primary response, naive B cells are activated by T cells and then move into germinal centers. A secondary response is driven by memory cells already in place. When these memory cells meet their specific antigen again, they rapidly proliferate and differentiate into plasma cells. For T cells, the response mediated by memory CD4 and CD8 cells is more rapid and aggressive than the primary response, and in one mouse study CD4 T cells also contracted more quickly during the secondary response.

Vaccination exploits this asymmetry. Spacing the priming doses typically 3–4 weeks apart allows antigen-specific T and B cells to mature and expand without competitive interference. Boosters have a measurable effect on plasma cell output: a transient plasmablast peak appears in the circulation a few days after each dose, and the second and third doses elicit earlier and more intense bursts. Where a booster is given also matters. A study of macrophage-dependent recall found that local boosting of the draining lymph node was more likely to recruit highly mutated, high-affinity memory B cells into secondary germinal centers than distal boosting on the opposite side. Taken together, these observations explain why vaccines are designed to generate both long-lived plasma cells and memory B cells, which together provide the basis for protection against re-exposure.

Identification and clinical measurement

Cluster of differentiation markers

Lymphocyte subsets are distinguished mainly by the proteins displayed on their surfaces. These proteins are named under the cluster of differentiation (CD) system, in which each number corresponds to a molecule recognized by a specific monoclonal antibody. CD3 is a component of the T-cell receptor complex and therefore marks essentially all T cells. CD4 and CD8 divide T cells into helper and cytotoxic or suppressor lineages, respectively. CD19 is expressed on B cells from early development until plasma-cell differentiation. Natural killer (NK) cells lack CD3 and are commonly identified by CD16 and CD56, usually in combination. Panels also include CD45, which is present on all leukocytes and is used to define the lymphocyte gate, and CD45RA, CD27, CD28 and CCR7, which separate naive, central-memory and effector-memory T cells. Regulatory T cells are typically defined by CD4 together with CD25, and confirmed by intracellular staining for FOXP3, because FOXP3 is not a surface marker.

Flow cytometry

Flow cytometry is the principal laboratory method for enumerating and phenotyping lymphocytes. Cells labelled with fluorescent antibodies are passed in single file through one or more lasers. The instrument records light scatter, which reflects cell size and granularity, and fluorescence from each marker. Analysts then apply sequential gates, first selecting leukocytes by CD45 and side scatter, then lymphocytes, then T, B and NK populations, and finally CD4 and CD8 subsets within T cells. Results are reported as percentages of a parent population, and are converted to absolute counts per microliter.

Two conversion strategies are in common use. In dual-platform testing, a separate hematology analyzer provides the absolute white-cell and lymphocyte counts, and the flow result supplies the proportions. In single-platform testing, a known number of fluorescent calibration beads is added to the sample, so absolute subset counts come directly from the flow run without a separate counter. The single-platform approach is generally regarded as more convenient and reproducible. Flow cytometry is especially important for monitoring CD4 counts in HIV infection, for assessing immune reconstitution after hematopoietic stem cell transplantation, and for diagnosing primary immunodeficiencies and characterizing lymphoid malignancies.

Absolute lymphocyte count in the complete blood count

The lymphocyte count is normally reported as part of a complete blood count (CBC) with differential. The laboratory expresses the result in two ways. The lymphocyte percentage is the share of lymphocytes among all white cells. The absolute lymphocyte count (ALC) is the number of lymphocytes per unit volume of blood. It is calculated by multiplying the total white-cell count by the lymphocyte fraction. For example, a white-cell count of 8,000 per microliter with 20% lymphocytes gives an ALC of 1,600 per microliter.

The absolute count is generally considered the more informative value. A normal percentage can conceal a low lymphocyte count when the total white-cell count is also low, and an elevated percentage may simply reflect neutropenia rather than an actual increase in lymphocytes. Historically, the ALC has also been used as a rough estimate of the CD4 count in resource-limited HIV care, although it is not a substitute for direct CD4 measurement. Persistently high counts are described as lymphocytosis, and low counts as lymphopenia.

Variability of reference ranges

No single reference interval applies to all people or all laboratories. Adult ALC intervals reported by different sources range from about 1,000 to 4,800 per microliter to about 700 to 3,100 per microliter, and the proportion of lymphocytes among white cells is commonly given as 20% to 40%. Children have substantially higher values. One widely cited set of ranges gives 3,000 to 9,500 per microliter for children, and neonates have very high counts relative to older children and adults.

Age also changes the subset composition. In infancy, absolute counts of T and B cells are higher than in adults, while the CD4:CD8 ratio falls steadily from roughly 2 in the first months of life to values typical of older children. In older adults, absolute counts tend to decline while the relative proportions of some subsets rise. Sex, ethnicity, altitude and lifestyle also influence values, and population-specific studies have found that ranges established in one region may differ from those in others. For example, a Hong Kong study reported adult 95% reference ranges of about 723 to 2,271 CD3⁺ cells per microliter and about 118 to 645 CD19⁺ cells per microliter, with significant differences by sex and age. Method differences add further variation, including whether a single-platform or dual-platform protocol is used, which antibody clones and gating strategies are applied, and how instruments are calibrated. For these reasons, laboratories are generally advised to establish or verify age- and method-specific reference intervals rather than relying on published values alone.

Disorders of lymphocyte number and function

Primary immunodeficiencies

Inherited defects in lymphocyte development show how much the adaptive immune system depends on these cells. Severe combined immunodeficiency (SCID) is the best-known example. Its incidence rate is estimated at 1/50,000 to 1/100,000 births. Before effective treatment, infants with the condition were profoundly lymphopenic and died of infection before their second birthday. Several hereditary forms exist, and they are linked to defects in at least 17 different genes.

The most common form is X-linked. It involves IL2RG, which encodes the interleukin common gamma chain, a component of the cytokine receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21. Because the shared subunit is needed for signals that T cells rely on to mature and expand, its loss leaves the T cell compartment underdeveloped. Females have two X chromosomes, so if a gene on one is mutated, the other healthy chromosome can compensate, which explains why the disease mostly affects males. A second major form, adenosine deaminase (ADA) deficiency, works differently. The enzyme also breaks down toxic metabolites that otherwise accumulate to harmful levels that kill lymphocytes. Defects in RAG1 and RAG2, which are required for V(D)J recombination, block the generation of functional receptors altogether. In early gene therapy trials for X-linked SCID, a retroviral vector restored T cell function, but five of those children developed leukemia after being treated in Paris and London, which led to redesigned vectors.

Secondary lymphopenia

Acquired loss of lymphocytes is far more common than inherited immunodeficiency. The clearest example is HIV infection, in which patients experience a progressive decline in CD4 T-cell number, resulting in immunodeficiency and increased susceptibility to opportunistic infections and malignancies. The mechanism is not fully resolved. One influential view held that the primary basis of T-cell depletion in patients infected with HIV is increased apoptosis of CD4 and CD8 T cells, although reduced production of new cells also contributes. Direct killing of infected cells by the virus does not appear sufficient on its own. Chronic immune activation alters the balance of the CD4 and CD8 pools, leading to CD4 T cell depletion and CD8 T cell expansion. Some people on effective antiretroviral therapy still fail to restore their CD4 counts. In these lymphopenic immunological non-responders, homeostatic proliferation is more prominent, and the surviving cells show signs of exhaustion and/or senescence. Other chronic infections, such as cytomegalovirus, can also cause lymphopenia through enhanced apoptosis, and cytotoxic cancer treatment (chemotherapy) depletes lymphocytes as a side effect.

Autoimmune disease

Autoimmunity reflects a failure of immune tolerance, in which lymphocytes that would attack the body's own tissues escape the checks of central and peripheral tolerance. In systemic lupus erythematosus (SLE), autoantibodies against nuclear antigens, immune-complex deposition, and resulting tissue inflammation are the hallmarks, most notably in the skin, joints, and kidneys. Most autoantibodies in human lupus are targeted against conserved nuclear antigens, such as DNA, nuclear proteins and nucleosomes. The initial trigger is unclear, but in both animal models and humans, central tolerance seems to remain intact, which points to a defect in peripheral control. B cells are central to the process. In SLE, the loss of B cell tolerance to autoantigens is controlled in a cell-intrinsic manner by Toll-like receptors (TLRs), which sense nucleic acids in endosomes. Many of the pathogenic autoantibodies are somatically hypermutated, class-switched IgGs, indicating that they were produced through germinal-center reactions. T cells also contribute, and substantial aberrations in peripheral T lymphocyte subsets are evident, and include a reduction of cytotoxic, regulatory, and advanced differentiated subtypes.

Type 1 diabetes follows a different pattern, with T cells destroying the insulin-producing beta cells of the pancreatic islets. Cytotoxic CD8 T cells and islet-reactive CD4 T cells are the main effectors, and autoantibodies to islet antigens serve as markers of the process. Genetic risk is strongly linked to HLA class II variants, and reduced regulatory T cell function is thought to contribute.

Lymphoid malignancies

Lymphomas and lymphocytic leukemias arise when a single lymphoid cell acquires malignant features and proliferates. Classification is based on the cell of origin and its stage of differentiation. Most non-Hodgkin lymphomas derive from B cells, and examples include follicular lymphoma, diffuse large B-cell lymphoma, and Burkitt lymphoma. Peripheral T-cell lymphomas form a smaller and more heterogeneous group. Chronic lymphocytic leukemia is an expansion of mature B cells, whereas acute lymphoblastic leukemia arises from precursor cells, most often of B lineage. Plasma cell neoplasms such as multiple myeloma are classed separately. Immunophenotyping by flow cytometry, along with cytogenetic findings such as the translocations that activate MYC in Burkitt lymphoma or BCL2 in follicular lymphoma, combine with lineage information to determine the diagnosis and prognosis.

Therapeutic targeting

Checkpoint blockade

Activated T cells express several coinhibitory receptors including PD-1 and CTLA-4. Antibodies against these molecules, which are classed as immune checkpoint inhibitors, work by lifting that restraint. CTLA-4 mainly governs the establishment of new immune responses, whereas PD-1 exerts its major influence on ongoing effector responses. PD-1 acts mainly on T cell activity in peripheral tissues through its interaction with PD-L1 and PD-L2.

The mechanisms differ. CTLA-4 is homologous to CD28 but has higher affinity for CD80 or CD86, so it can out-compete the costimulatory receptor for the same ligands. On the PD-1 side, recruitment of the phosphatase SHP2 directly attenuates TCR signaling by dephosphorylating proximal signaling elements. Chronically stimulated T cells end up in a state of exhaustion, and the intensity of PD-1 expression increases progressively during this exhaustion. Blocking the receptor or its ligand is intended to reawaken tumour-infiltrating CD8 T cells. Humanized monoclonal antibodies against CTLA-4, PD-1 and PD-L1 are now used alone or combined with other treatments across a range of cancers. Antibodies with suitable Fc isotypes can also act on regulatory cells: optimizing the Fc domain of anti-CTLA-4 antibodies can enhance antibody-dependent cellular cytotoxicity, thereby depleting regulatory T cells in the tumour microenvironment. Because the brakes are released broadly rather than only in tumours, checkpoint inhibitors modulate overall immune activity and can cause side effects ranging from moderate to fatal. These immune-related effects are closely tied to autoimmunity.

Adoptive cell therapy and CAR T cells

Chimeric antigen receptor (CAR) T-cell therapy is a form of cancer-immunotherapy in which the patient's own cells are reprogrammed outside the body. The patient's lymphocytes are collected by apheresis and genetically modified so they express a synthetic receptor, the "chimeric antigen receptor", that recognises tumour antigens. After expansion in the laboratory, the modified cells are infused back, combining the antigen-binding properties of antibodies with the effector functions of lymphocytes. The approach has been applied to relapsed or refractory haematologic malignancies such as multiple myeloma, B-cell lymphoma and acute lymphoblastic leukemia. Many constructs have been directed at CD19; for example, trials have examined CD19-targeted CAR T-cell therapy for relapsing lymphoma.

Toxicity: cytokine release syndrome and neurotoxicity

The most frequent toxicity is cytokine release syndrome (CRS). It is a systemic inflammatory response caused by overactivation of effector cells and large amounts of cytokines. The elevated cytokines include interferon-gamma, GM-CSF, IL-10 and IL-6. Clinically this can progress from fever and hypotension to organ dysfunction, respiratory failure and coagulopathy, and CRS can be fatal. Neurological toxicity, called immune effector cell-associated neurotoxicity syndrome (ICANS), is a separate problem. It is an inflammatory neurotoxicity that typically occurs after CRS and is characterized by impaired blood-brain barrier integrity. It often appears even after the initial CRS symptoms have subsided. Management remains difficult: ICANS is currently managed with corticosteroids, although the optimal dose and duration are still undetermined. Research into the efficacy of interleukin inhibitors in addressing CRS and neurotoxicity is one line of effort to moderate these reactions.

Monoclonal antibodies in cancer and autoimmune disease

Antibodies against lymphocyte surface molecules also serve as therapy. The prototype is rituximab, an anti-CD20 antibody. It binds CD20, a B cell surface marker, and leads to cell death through complement-induced lysis and apoptosis. Introduced in 1997, it has since received FDA approval for non-Hodgkin lymphoma, chronic lymphocytic leukemia, rheumatoid arthritis and several vasculitides. In autoimmune disease, the logic is that CD20 is lost from B cells during their terminal differentiation into plasma cells, so many pathogenic plasma cells may not be effectively removed and may continue to produce antibodies. In lupus, B cell depletion with rituximab alone, or combined with cyclophosphamide and corticosteroids, has been reported to benefit some patients with severe systemic lupus erythematosus, although these were open-label studies. Second-generation agents such as ocrelizumab, obinutuzumab, veltuzumab and the fully human ofatumumab were subsequently developed with the aim of greater effectiveness and lower immunogenicity.

Discovery and research history

For much of the nineteenth and early twentieth centuries, lymphocytes were known only as small round cells that filled the blood and lymphoid organs, and their function was unclear. Recognizing them as a separate population was a gradual process of microscopic description, and the question of what they did only became answerable after experiments in whole animals.

The bursa, the thymus, and two lineages

The bursa of Fabricius, a hindgut lymphoepithelial organ in birds, was described in lecture notes published in 1621, long before its immunological role was known. In 1956 Bruce Glick and Timothy Chang, two graduate students at Ohio State University, found that removing the bursa from newly hatched chickens sharply reduced their antibody production in adulthood. Their finding was published in a poultry journal and remained largely unnoticed for several years. Glick and a fellow graduate student, Timothy Chang, serendipitously discovered the function of the bursa while preparing an undergraduate immunology laboratory. Glick published his results in 1956; however, they remained obscure for another 5 years.

Around the same time, Jacques Miller's work on the thymus established a second population. In 1958 Francis A. P. Miller in Australia discovered the role of thymus-derived cells in cellular immunity, and his thymectomy experiments showed that removing the organ early in life depleted lymphocytes in other lymphoid organs. Max Dale Cooper's later chicken experiments, which used irradiation to clear pre-existing cells, separated the two systems: the antibody producing lineage, derived in chickens from the bursa of Fabricius and named B cells, and the lineage responsible for cell-mediated immunity arising from the thymus and given the name T cells. Ivan Roitt and colleagues in London reviewed these data and introduced the terms B and T lymphocytes in 1969. Because the bursa is found only in birds, mammalian B cell development was later traced to bone marrow.

Clonal selection

The theoretical basis for lymphocyte specificity came from antibody theory. In 1897, Paul Ehrlich proposed that cells had multiple receptors called 'side chains', which were able to bind with whichever antigen came along. Niels Jerne's 1955 proposal that antibody specificities already exist before antigen exposure, and that antigen selects among them, was a key step. In 1957, Frank Macfarlane Burnet published his version of this idea in an obscure Australian journal, expanding the ideas of Talmage and naming the resulting theory the "clonal selection theory". David Talmage had proposed a similar model in the same year. Burnet also argued that generating such diversity required a cellular mechanism, and he set out the framework in his 1959 book. The idea that specificity for diverse antigens exists before these antigens are encountered was a radical notion at the time, but one that became widely accepted. The theory also explained immunological memory as the expansion of long-lived clones, and it linked deletion of self-reactive clones to tolerance. Clonal selection thus provided the conceptual framework for later work on tolerance and memory.

Cloning the T cell receptor

Demonstrating that lymphocytes carry diverse receptors at the molecular level took longer. Early evidence came from antibody-like polypeptides on T cell clones, but the genes proved hard to isolate. In March 1984, back-to-back papers reporting the discovery of the genes encoding the mouse and human TCR were finally published in the March 8, 1984 edition of Nature. Those reports, from the group led by Mark Davis, described the beta-chain genes, and related studies from the same period described the alpha chain. The alpha chain work showed that the α- and β-chains are composed of variable (V) and conserved (C) regions, and that gene segments encoding variable and conserved domains of the β-chain have been identified and undergo rearrangements during T-cell differentiation. These findings linked the receptor to the same kind of gene rearrangement that generates antibody diversity. Cloning in other species followed; a chicken TCR beta cDNA showed that TCR beta mRNA was expressed in cells isolated from the thymus but not in cells from the bursa of Fabricius, which supports the thymic origin of T cells.

Cell-surface markers and analytical tools

Once antibodies could be raised against specific cell-surface proteins, the classification of lymphocytes became a matter of marker identification. Monoclonal antibodies, developed in 1975, allowed researchers to define populations that could not be separated by appearance alone. Standardized naming through cluster of differentiation (CD) numbers, adopted in the early 1980s, and flow cytometry, which measures marker expression on individual cells, turned these antibodies into a routine method for identifying lymphocyte subsets. Together these technologies moved lymphocyte research from morphology to molecular phenotype, and they remain central to diagnosis and to the study of T cell receptor diversity.

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