Antibodies

Antibodies, or immunoglobulins, are large glycoprotein molecules produced primarily by plasma cells that circulate in blood and extracellular fluids. They recognize specific antigens with high affinity and specificity and help defend the body against pathogens.

Each antibody is composed of two identical heavy chains and two identical light chains, held together by disulfide bonds. The tips of the Y, the variable regions of the chains, form the antigen-binding sites (Fab). The stem, the constant region (Fc), interacts with immune effector cells and complement proteins, shaping the antibody's distribution and function.

Diversity is generated by somatic recombination of gene segments (V(D)J), junctional diversity, and somatic hypermutation, enabling the vast repertoire needed to recognize countless antigens. Antibodies exist in several isotypes—IgM, IgD, IgG, IgA, and IgE—each with distinct locations and roles.

Functions include neutralizing pathogens and toxins by binding to them; opsonizing targets to enhance phagocytosis via Fc receptors; activating the classical complement pathway; mediating antibody-dependent cellular cytotoxicity (ADCC); and providing mucosal defense (IgA) or mediating allergic responses (IgE). Binding to antigens can cause cross-linking and immune complex formation, contributing to pathogen clearance but potentially to pathology if uncontrolled.

Antibodies are produced by B cells that differentiate into plasma cells. Polyclonal antibodies arise from many clones; monoclonal antibodies come from a single clone (traditionally via hybridoma technology) and can be produced recombinantly. Therapeutic antibodies are often engineered to be human or humanized to reduce immunogenicity.

Clinical uses include diagnostics (immunoassays), therapeutics for cancer, autoimmune and infectious diseases, and research applications. Key concepts also include affinity, avidity, and half-life, influenced by the Fc region and neonatal Fc receptor (FcRn).