Antigen

An antigen is a molecule or molecular structure specifically recognized by an antibody, B-cell receptor, or T-cell receptor. Recognition can trigger or guide an adaptive immune response.

Epitope: The specific part of an antigen bound by an antibody or recognized by an immune receptor. Antibodies and receptors bind particular epitopes rather than every part of an antigen.
Immunogen: A substance capable of inducing an adaptive immune response. Every immunogen is antigenic, but some recognized antigens do not induce a response.
Vaccine: A preparation that trains the immune system to recognize a pathogen or other target without causing the disease it prevents. Vaccines expose immune systems to selected antigens to build protective memory.
Autoantigen: A body's own molecule that is targeted by an adaptive immune response. Autoantigens are self molecules whose recognition contributes to autoimmune disease.
Antigen presentation: The display of peptide fragments on cell-surface molecules for recognition by T cells. Most T cells recognize antigen only after cells display its fragments.
Hapten: A small molecule that can bind antibodies but usually becomes immunogenic only when attached to a larger carrier. Haptens show that recognition alone may not be enough to provoke immunity.
Serology: The study and testing of blood serum for antibodies and other immune markers. Serological tests often infer exposure by detecting antibodies against particular antigens.
Superantigen: A microbial protein that activates many T cells by binding outside the usual antigen-specific recognition site. Superantigens bypass ordinary specificity and can provoke unusually broad T-cell activation.
Major histocompatibility complex: Cell-surface proteins that present peptide antigens to T cells and help distinguish self from nonself. These molecules hold peptide antigens in a form T-cell receptors can inspect.
Carrier protein: A protein that helps a linked small molecule provoke an immune response. Carrier proteins can make otherwise weakly immunogenic haptens effective immunogens.

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