An antibody is a protein of the immunoglobulin family produced by B cells of the immune system. It binds particular features of an antigen, enabling recognition of microorganisms, toxins, and other molecular targets. Antibodies occur as membrane-bound antigen receptors and as soluble molecules secreted principally by plasma cells, differentiated descendants of B cells. They are central components of adaptive immunity, linking specific recognition to mechanisms that neutralize targets or promote their removal. (ncbi.nlm.nih.gov)
Structure and antigen recognition
A typical antibody, exemplified by immunoglobulin G (IgG), consists of two identical heavy chains and two identical light chains connected by disulfide bonds. Together they form a roughly Y-shaped structure. Each chain contains variable and constant regions. The paired variable regions at each arm’s tip form an antigen-binding site; the heavy-chain constant regions determine the antibody’s class and many of its biological properties. Light chains occur in two types, kappa and lambda. (ncbi.nlm.nih.gov)
The two arms contain the Fab regions, responsible for antigen binding, while the stem contains the Fc region, which interacts with immune receptors and other effector molecules. A flexible hinge in IgG permits the arms to adopt different orientations. This arrangement separates target recognition from the recruitment of immune functions. (ncbi.nlm.nih.gov)
An antibody recognizes an epitope, a particular portion of an antigen, rather than necessarily recognizing the entire molecule. Its binding surface is shaped by highly variable stretches of amino-acid sequence. Binding depends on complementary molecular surfaces and noncovalent interactions. Affinity describes the strength of an individual binding interaction; avidity describes the combined strength of multiple interactions. Specificity is not absolute: an antibody may cross-react with different antigens that present sufficiently similar features. (ncbi.nlm.nih.gov)
Antibody classes
Humans have five major immunoglobulin classes, or isotypes, distinguished by their heavy chains: IgG, IgA, IgM, IgE, and IgD. Their distributions and effector properties differ. (ncbi.nlm.nih.gov)
- IgG is the most abundant immunoglobulin in blood plasma. It supports neutralization, target clearance, and other defensive functions. Maternal IgG crosses the placenta, providing passive protection to the developing fetus and newborn.
- IgA is important at mucosal surfaces and occurs in secretions such as saliva, tears, and milk. Blood IgA is mainly monomeric, whereas secretory IgA commonly consists of two antibody units.
- IgM is typically the first class secreted during a primary antibody response. Secreted IgM is predominantly pentameric, allowing strong multivalent binding and efficient complement activation; membrane-bound IgM is monomeric.
- IgE participates in responses to parasitic worms and in allergic reactions through interactions with cells including mast cells.
- IgD is principally associated with antigen recognition on mature naive B cells, which commonly express both IgM and IgD. Its concentration in blood is low. (ncbi.nlm.nih.gov)
Generation of diversity and immune responses
Antibody diversity arises partly through V(D)J recombination. During B-cell development, segments of DNA encoding variable regions are rearranged: heavy chains use variable, diversity, and joining segments, while light chains use variable and joining segments. Variation at segment junctions and different heavy–light chain pairings further expand the repertoire. Thus, a limited collection of inherited gene segments can generate many antigen-binding structures. (ncbi.nlm.nih.gov)
After antigen recognition and appropriate activation signals, selected B cells proliferate and differentiate into antibody-secreting cells. Responses to many protein antigens involve help from T cells. Activated B cells can undergo somatic hypermutation, introducing mutations into variable-region sequences. Selection of cells whose receptors bind antigen more effectively produces affinity maturation. (ncbi.nlm.nih.gov)
Class-switch recombination changes the heavy-chain constant region, allowing a B-cell lineage to produce a different isotype without replacing its rearranged antigen-binding variable region. This alters effector properties rather than, by itself, changing antigen specificity. Membrane-bound and secreted forms are also generated through differences in RNA processing. (ncbi.nlm.nih.gov)
Effector functions
Antibodies protect through several complementary mechanisms. Neutralization blocks a pathogen or toxin from interacting productively with host cells. Binding alone does not necessarily imply neutralization: the location and consequences of binding matter. Opsonization coats a target with antibodies, facilitating recognition by immune cells and its uptake through phagocytosis. (ncbi.nlm.nih.gov)
Certain antibody classes also activate the complement system, a network of plasma proteins that promotes target clearance and can damage susceptible microbial membranes. These mechanisms connect antigen-specific recognition to effector systems that also participate in innate immunity. Antibody activity therefore depends not only on binding specificity but also on isotype, target accessibility, and the available cellular machinery. (ncbi.nlm.nih.gov)
Laboratory and therapeutic applications
Antibodies are widely used as molecular recognition reagents. Enzyme-linked immunosorbent assays use antigen–antibody binding and an enzyme-generated signal to detect or quantify substances. Depending on the assay design, the measured target may be an antigen or an antibody. Such tests are used in research and diagnostic laboratories, including the measurement of antibodies against infectious agents and autoantibodies. (ncbi.nlm.nih.gov)
A monoclonal antibody preparation has a defined binding specificity, whereas polyclonal preparations contain antibodies from multiple B-cell lineages. Therapeutic monoclonal antibodies are laboratory-produced proteins designed to bind particular targets. In cancer treatment, different antibodies can block growth-related signals, recruit immune destruction, or deliver a linked toxic payload. Antibody–drug conjugates combine an antibody’s targeting function with a cell-killing substance; their biological action depends on both components. (ncbi.nlm.nih.gov)