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.
Isoantibodies
Isoantibodies (formerly called alloantibody) are antibodies produced by an individual against isoantigens produced by members of the same species. In the case of the species "Homo sapiens", for example, there are a significant number of antigens which are different in every individual. When antigens from another individual are introduced into another's body, these isoantibodies immediately bind to and destroy them.