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.
Cancer immunoprevention
Two main protective mechanisms elicited by cancer immunoprevention in various mouse models were cytokines released by T cells, in particular gamma-interferon, and cytotoxic antibodies against the target antigen. This is at variance with cancer immunotherapy administered to cure existing tumors, which is mainly based on cytotoxic T lymphocytes (CTL). The lack of a relevant CTL response in long-term immunoprevention is thought to be an advantage, because chronic CTL activation is severely toxic for the host. In contrast circulating antibodies provide long-term protection without toxic side effects. A similar situation happens in viral immunity, acute infections are resolved by CTL, whereas long term immunity from reinfection is provided by antibodies.
Both gamma-interferon and antibodies prevent tumor growth in multiple ways. Gamma-interferon activates T, natural killer and B cells, inhibits angiogenesis and tumor invasiveness, stimulates major histocompatibility complex expression in tumor cells and inhibits cell proliferation. Antibodies binding to antigens on the surface of cells trigger lytic mechanisms mediated by the complement system (complement-mediated cytotoxicity) or by leukocytes carrying Fc receptors (antibody-dependent cell-mediated cytotoxicity, ADCC). Moreover, antibody binding interferes with the cellular functions of the target antigen, causing its internalization or hampering molecular interactions, eventually blocking downstream signaling. If the target antigen controls cell growth (e.g. if it is the product of an oncogene), then a block of signaling can disrupt the carcinogenic process. Surface antigens causally involved in carcinogenesis are called oncoantigens.