Antigen
In immunology, antigens (Ag) are structures (aka substances) specifically bound by antibodies (Ab) or a cell surface version of Ab ~ B cell antigen receptor (BCR). The term antigen originally described a structural molecule that binds specifically to an antibody only in the form of native antigen. It was expanded later to refer to any molecule or a linear molecular fragment after processing the native antigen that can be recognized by T-cell receptor (TCR). BCR and TCR are both highly variable antigen receptors diversified by somatic V(D)J recombination. Both T cells and B cells are cellular components of adaptive immunity. The Ag abbreviation stands for an antibody generator.
Autoimmune disease
Although it is possible for a potential autoantigen to be spatially sequestered in an immune privileged site within the body (e.g. the eye), mechanisms exist to express even these antigens in a tolerogenic fashion to the immune system. However, it is impossible to induce tolerance (immune unresponsiveness) to all aspects of an autoantigen. This is because under normal physiologic conditions some regions of a self-antigen are not expressed at a sufficient level to induce tolerance. These poorly displayed areas of an antigen are called "cryptic determinants." The immune system maintains a high-affinity repertoire to the cryptic self because the presentation of these determinants was insufficient to induce strong tolerance.