<P> 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 peptides (amino acid chains), polysaccharides (chains of monosaccharides / simple sugars) or lipids . 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 . The term antigen originally described a structural molecule that binds specifically to an antibody . It was expanded to refer to any molecule or a linear molecular fragment that can be recognized by highly variable antigen receptors (B - cell receptor or T - cell receptor) of the adaptive immune system . </P> <P> The antigen may originate from within the body ("self - antigen") or from the external environment ("non-self"). The immune system usually does not react to self - antigens under normal homeostatic conditions due to negative selection of T cells in the thymus and is supposed to identify and attack "non-self" invaders from the outside world or modified / harmful substances present in the body under distressed conditions . </P> <P> Antigen presenting cells present antigens in the form of peptides on histocompatibility molecules . The T cell / T lymphocyte (a subtype of white blood cell), of the adaptive immune system, selectively recognize the antigens . Depending on the antigen and the type of the histocompatibility molecule, different types of T cells will be activated . For T - Cell Receptor (TCR) recognition, the peptide must be processed into small fragments inside the cell and presented by a major histocompatibility complex (MHC). The antigen cannot elicit the immune response without the help of an immunologic adjuvant . Similarly, the adjuvant component of vaccines plays an essential role in the activation of the innate immune system . </P> <P> An immunogen is an antigen substance (or adduct) that is able to trigger a humoral (innate) or cell - mediated immune response . It first initiates an innate immune response, which then causes the activation of the adaptive immune response . An antigen binds the highly variable immunoreceptor products (B - cell receptor or T - cell receptor) once these have been generated . Immunogens are those antigens, termed immunogenic, capable of inducing an immune response . </P>

Recognition proteins are used by white blood cells for what purpose