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.
Experiments in immunology
However, some antibodies found in antiserum are cross-reactive. This means that they bind antigens that have no apparent relationship to the immunogen, and are problematic when antiserum is used to detect antigens that bind specifically to these antibodies. This can be resolved by absorption of these antibodies with their cross-reactive antigen, leaving only the antibodies that are specific to the immunogen of interest.