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.
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.