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.
Human blood group systems
In addition to the ABO and Rh systems, the antigens expressed on blood cell membrane surfaces include 346 red blood cell antigens and 33 platelet antigens, as defined serologically. The genetic basis for most of these antigens lie in 45 red blood cell and 6 platelet genes. An individual, for example, can be AB RhD positive, and at the same time M and N positive in the MNS system, K positive in the Kell system, and Le or Le positive in the Lewis system, where these and many of the systems are named for patients in whom the corresponding antibodies were first detected.