Antibodies are specialised proteins produced by B cells after their differentiation into antibody-secreting plasma cells. They belong to the immunoglobulin family and recognise particular molecular structures, called epitopes, on antigens. Each antibody has a variable region whose three-dimensional structure determines what it can bind and relatively conserved constant regions that determine many of its biological effects. Antibodies are central components of adaptive humoral immunity because they can recognise extracellular pathogens, toxins and other foreign molecules with high specificity. They can also recognise altered self molecules in some diseases. An antibody is not itself a cell and does not normally destroy a pathogen simply by binding it; rather, antibody binding can neutralise targets or recruit other components of the immune system.
The basic antibody molecule consists of two identical heavy chains and two identical light chains joined by disulphide bonds. Each chain contains variable and constant domains, and the variable portions together form antigen-binding sites. The antigen-binding fragment is commonly called the Fab region, whereas the stem-like Fc region interacts with Fc receptors on immune cells and with components of the complement system. The enormous diversity of antibodies is generated principally through V(D)J gene recombination in developing B cells, together with junctional diversity and subsequent mechanisms that can alter antibody affinity. This allows the immune system to generate receptors capable of recognising an extraordinarily large range of molecular structures.
Humans have five major antibody classes: IgM, IgG, IgA, IgE and IgD. IgM is commonly the first major antibody class produced during a primary immune response and is particularly effective at activating complement; secreted IgM commonly forms pentamers. IgG is abundant in blood and tissues and has important roles in neutralisation, opsonisation and complement activation, with different IgG subclasses having different properties. IgA is particularly important at mucosal surfaces and is commonly secreted as a dimer in secretions. IgE participates in responses involving mast cells and basophils and is important in allergic disease and defence against some parasites. IgD is expressed mainly on the surface of mature naive B cells and has a less clearly defined role as a soluble antibody.
Antibodies protect against infection through several mechanisms. Neutralising antibodies can bind viruses, toxins or microbial surface structures and prevent them from attaching to or entering host cells. Opsonising antibodies can coat a pathogen and make it easier for phagocytic cells bearing appropriate Fc receptors to recognise and ingest it. Antibody binding can also activate the classical complement pathway, producing downstream effects that include enhanced inflammation, opsonisation and, for susceptible organisms, membrane attack complex formation. Antibodies can additionally participate in antibody-dependent cellular cytotoxicity, in which cells such as Natural Killer Cells recognise antibody-coated targets through Fc receptors and contribute to their destruction.
During an immune response, B-cell clones can undergo class-switch recombination and somatic hypermutation. Class switching changes the antibody constant region while retaining antigen specificity, allowing the same basic recognition specificity to acquire different effector properties. Somatic hypermutation introduces mutations into immunoglobulin variable-region genes, and selection of B cells expressing higher-affinity antibodies contributes to affinity maturation in germinal centres. These processes are supported by interactions with T follicular helper cells and other components of the lymphoid environment. They explain why later antibody responses can be more rapid, abundant and higher affinity than the initial response to the same antigen.
Antibodies are also used extensively outside normal immunity. Laboratory immunoassays exploit antibody specificity to detect hormones, proteins, pathogens, drugs and other molecules. Monoclonal antibodies are produced from a single B-cell lineage and are designed to recognise a defined target, making them useful as research reagents, diagnostic tools and therapeutic medicines. Therapeutic antibodies can block receptors or signalling molecules, activate or inhibit immune mechanisms, deliver cytotoxic agents, or alter immune-cell behaviour. Antibody-based treatments therefore range from passive protection against particular infections to therapies for cancers, inflammatory diseases and other disorders.
Antibodies should not be confused with antigens. An antigen is a molecule or molecular structure that can be specifically recognised by an immune receptor, whereas an antibody is one type of immune molecule that recognises particular antigenic structures. An epitope is the specific portion of an antigen recognised by an antibody or receptor. Antigenicity describes the capacity to be specifically recognised, whereas immunogenicity concerns the capacity to induce an immune response. These concepts are related but not interchangeable. A molecule can sometimes be recognised without producing a strong immune response, which is one reason why antigenicity and immunogenicity require separate definitions.
Antibodies contribute to immune memory but are not identical to immune memory itself. Long-lived plasma cells can continue producing antibodies for extended periods, while memory B cells remain available to respond to later exposure. Vaccination exploits these mechanisms by exposing the immune system to an appropriate antigenic stimulus without requiring the person to experience the target disease in its natural form. Antibody concentrations can decline over time, so protection against some infections depends on persistent antibody production, memory responses, repeated vaccination or combinations of these mechanisms. Antibodies can also become harmful when they recognise self structures, form immune complexes excessively or participate in inappropriate allergic reactions.
