Antigeen

An antigen is any molecule or part of a molecule that can be recognized by the immune system. Most antigens are proteins or polysaccharides, but they can also be lipids or nucleic acids. The specific portion that is recognized by B cell receptors or T cell receptors is called an epitope. Antigens differ in size, complexity, and chemical composition, all of which influence their immunogenicity—the ability to provoke an immune response—and their antigenicity, the ability to be recognized by immune receptors.

Processing and presentation: B cells can recognize intact antigens via their B cell receptor and may internalize them for presentation to helper T cells. T cells, by contrast, recognize peptide fragments that have been processed inside antigen-presenting cells and presented on MHC molecules. Exogenous antigens are typically presented on MHC class II to CD4+ T cells, whereas endogenous antigens are presented on MHC class I to CD8+ T cells.

Origins and special cases: Antigens can be foreign, such as components of pathogens or vaccines; endogenous antigens come from within infected or abnormal cells; autoantigens are host proteins that can trigger autoimmune reactions. Small molecules called haptens are antigenic but not immunogenic on their own; they become immunogenic when bound to a larger carrier protein.

Clinical relevance: The study of antigens underpins vaccine development, diagnostics, and immunotherapies. Allergens are antigens that provoke hypersensitivity reactions, while tolerance mechanisms normally prevent responses to self-antigens. Adjuvants are substances that enhance the immune response to an antigen.