Adaptive immunity is the component of the immune system that develops targeted responses to particular antigens through the activity of B lymphocytes and T lymphocytes. Its defining features include a highly diverse repertoire of antigen receptors, selective multiplication of responding cells, and immunological memory. It works alongside innate immunity, which supplies rapid defense and signals that help initiate adaptive responses. An initial adaptive response usually takes several days to develop, whereas subsequent encounters with the same antigen can elicit faster and more effective responses. (ncbi.nlm.nih.gov)
Cells and lymphoid organs
B and T lymphocytes originate from blood-forming stem cells. In humans, B cells undergo their early development in the bone marrow, while T-cell precursors migrate to the thymus to mature. During development, each population acquires specialized antigen receptors and undergoes selection that shapes its functional repertoire. Mature cells that have not yet encountered their corresponding antigen are called naive lymphocytes. (ncbi.nlm.nih.gov)
Naive lymphocytes circulate through blood and tissues of the lymphatic system. Responses are commonly initiated in secondary lymphoid organs, including lymph nodes, the spleen, and mucosal lymphoid tissues. These organs bring lymphocytes into contact with antigens and antigen-presenting cells. Dendritic cells are particularly important for initiating naive T-cell responses: they acquire antigen in tissues and present it together with activating signals. This organization allows relatively rare antigen-specific lymphocytes to encounter their targets efficiently. (ncbi.nlm.nih.gov)
Recognition and receptor diversity
B-cell receptors bind directly to accessible structures on antigens, including particular regions called epitopes. The receptors are membrane-bound immunoglobulins; their secreted counterparts are antibodies. Conventional T-cell receptors instead recognize peptide fragments displayed by major histocompatibility complex (MHC) molecules. Consequently, B and T cells recognize antigen in different forms, even when responding to the same microorganism. (ncbi.nlm.nih.gov)
Receptor diversity arises largely through V(D)J recombination, which assembles receptor-coding sequences from separate gene segments during lymphocyte development. Variation at segment junctions and pairing of receptor chains further enlarge the repertoire. These changes occur in lymphocytes rather than in reproductive cells and are not inherited by offspring. The initial repertoire therefore exists before exposure to the antigens that will later select responding cells. (ncbi.nlm.nih.gov)
Activation and clonal selection
The organizing principle of adaptive responses is clonal selection. Antigen recognition selects lymphocytes with suitable pre-existing receptors; appropriate additional signals stimulate them to proliferate and differentiate. This clonal expansion produces enough antigen-specific cells to perform protective functions. Antigen does not instruct the immune system to construct a receptor from scratch. (ncbi.nlm.nih.gov)
For naive T cells, recognition of peptide–MHC generally must be accompanied by costimulation. Cytokines help guide proliferation and differentiation into functionally distinct populations. These additional signals connect adaptive activation to the conditions detected by innate immune cells and reduce the likelihood that antigen recognition alone will provoke an inappropriate response. Many B-cell responses likewise require assistance from activated helper T cells, although some antigens can stimulate B cells without that help. (ncbi.nlm.nih.gov)
Humoral and cell-mediated responses
Humoral immunity is mediated principally by antibodies secreted by differentiated B cells called plasma cells. Antibodies can neutralize toxins and block microbial attachment or entry into host cells. They also mark targets for phagocytosis and can activate the complement system. Antibody recognition thus recruits mechanisms shared with innate defense rather than acting as an entirely separate protective system. (ncbi.nlm.nih.gov)
During many B-cell responses, somatic hypermutation introduces changes into antibody variable-region genes. Selection of cells whose receptors bind antigen more effectively produces affinity maturation. Class-switch recombination changes the antibody heavy-chain constant region, allowing the same antigen-binding specificity to be associated with different biological functions. These processes are distinct from the receptor rearrangements that generate the initial repertoire. (ncbi.nlm.nih.gov)
Cell-mediated immunity involves T-cell effector functions. CD4 helper T cells coordinate other immune cells, including B cells and macrophages. CD8 cytotoxic T cells recognize appropriate peptide–MHC class I complexes and can induce death in infected target cells. Antibody-mediated and T-cell-mediated responses frequently cooperate during the same infection, with their relative contributions depending on the pathogen and its location. (ncbi.nlm.nih.gov)
Memory and vaccination
After antigen clearance, many expanded effector lymphocytes die through apoptosis, while memory populations persist. Memory B and T cells support enhanced responses upon renewed exposure; persistent antibody production also contributes to protection. The duration and effectiveness of protection differ among antigens and infections, so immunological memory does not invariably prevent reinfection. (ncbi.nlm.nih.gov)
A vaccine induces active immunity by exposing the immune system to antigen in a controlled form. Passive immunity instead results from receiving antibodies produced elsewhere, as occurs with maternal antibody transfer. It can provide immediate protection but does not itself establish the recipient’s antigen-specific memory response. (cdc.gov)
Tolerance and immune dysfunction
Immune tolerance restrains responses against the body’s own constituents. Developmental selection eliminates or alters many self-reactive lymphocytes, while peripheral mechanisms inactivate, delete, or suppress others. Regulatory T cells contribute to this suppression. Failures of these controls can permit autoimmunity, whereas inadequate adaptive responses can contribute to immunodeficiency. Adaptive recognition can also cause harmful responses to transplanted tissue or participate in allergic reactions. (ncbi.nlm.nih.gov)