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/testis antigens
Cancer/testis (CT) antigens are a group of proteins united by their importance in development and in cancer immunotherapy. In general, expression of these proteins is restricted to male germ cells in the adult animal. However, in cancer these developmental antigens are often re-expressed and can serve as a locus of immune activation. Thus, they are often classified as tumor antigens. The expression of CT antigens in various malignancies is heterogeneous and often correlates with tumor progression. CT antigens have been described in melanoma, liver cancer, lung cancer, bladder cancer, and pediatric tumors such as neuroblastoma. Gametogenesis offers an important role for many of these antigens in the differentiation, migration, and cell division of primordial germ cells, spermatagonia spermatocytes and spermatids.