Antigen
Also, an antigen is a molecule that binds to Ag-specific receptors, but cannot necessarily induce an immune response in the body by itself. Antigens are usually proteins, peptides (amino acid chains) and polysaccharides (chains of monosaccharides/simple sugars) but lipids and nucleic acids become antigens only when combined with proteins and polysaccharides. In general, saccharides and lipids (as opposed to peptides) qualify as antigens but not as immunogens since they cannot elicit an immune response on their own. Furthermore, for a peptide to induce an immune response (activation of T-cells by antigen-presenting cells) it must be a large enough size, since peptides too small will also not elicit an immune response.
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.