antigen

An antigen is any substance that can be recognized by the immune system and potentially provoke an adaptive immune response. Antigens are typically proteins, polysaccharides, lipids, or nucleic acids, or parts of these molecules. The specific region of an antigen recognized by an antibody or T cell receptor is called an epitope.

An antigen may be immunogenic, able to induce an immune response, or merely antigenic, able to bind to antibodies or receptors without triggering a full response. Haptens are small molecules that are antigenic only when linked to a larger carrier protein or to a cell surface molecule.

Antigen processing and presentation are central to adaptive immunity. Antigen-presenting cells process proteins into peptides and display them on MHC molecules to T cells. B cells can bind native antigens through their B cell receptors and differentiate into antibody-secreting cells. Antigens are classified as exogenous (originating outside the body), endogenous (generated within cells, often in the context of infection or transformation), or autoantigens (derived from self molecules).

Antibody binding and T cell recognition form the basis of many immune responses. Allergens are antigens that trigger hypersensitivity in susceptible individuals. Vaccines use defined antigens or antigenic fragments to elicit protective immunity. In diagnostics, antigen–antibody interactions underpin serology and rapid tests.