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.
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.