In immunology, an antigen (Ag) is a molecule or molecular structure, such as may be present on the outside of a pathogen, that can be bound by an antigen-specific antibody or B cell antigen receptor. The presence of antigens in the body normally triggers an immune response. The Ag abbreviation stands for an antibody generator.Antigens are "targeted" by antibodies. Each antibody is specifically produced by the immune system to match an antigen after cells in the immune system come into contact with it; this allows a precise identification or matching of the antigen and the initiation of an adaptive response. The antibody is said to "match" the antigen in the sense that it can bind to it due to an adaptation in a antigen-binding fragment of the antibody. In most cases, an adapted antibody can only react to and bind one specific antigen; in some instances, however, antibodies may cross-react and bind more than one antigen.
Antigens are proteins, peptides (amino acid chains) and polysaccharides (chains of monosaccharides/simple sugars) but lipids and nucleic acids become antigens only when combined with proteins and polysaccharides.The antigen may originate from within the body ("self-antigen") or from the external environment ("non-self"). The immune system identifies and attacks "non-self" external antigens and usually does not react to self-antigens due to negative selection of T cells in the thymus.Vaccines are examples of antigens in an immunogenic form, which are intentionally administered to a recipient to induce the memory function of the adaptive immune system towards antigens of the pathogen invading that recipient. The seasonal flu virus is a common example.