Antigen
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 a tailored response. The antibody is said to "match" the antigen in the sense that it can bind to it due to an adaptation in a region of the antibody; because of this, many different antibodies are produced, each able to bind a different antigen while sharing the same basic structure. 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.
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.