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.
Oncoantigen
Localization of oncoantigens outside tumor cells allows recognition by antibodies if downregulation of class I major histocompatibility complex (MHC-I) molecules prevents T cell recognition. Most tumor antigens are intracellular proteins. Circulating antibodies do not penetrate inside cells, hence intracellular proteins are only recognized by T cells as MHC-I-bound antigenic peptides exposed on the surface of tumor cells. However downmodulation or complete loss of MHC-I expression occurs in most human tumors, making them altogether invisible to the immune system of the host. When tumor cells downregulate MHC-I, only antigens expressed on the cell surface and/or secreted in the extracellular fluids can be recognized by antibodies.