In immunology, an antigen is a molecule capable of inducing an immune response (to produce an antibody) in the host organism.Sometimes antigens are part of the host itself in an autoimmune disease
Antigens are “targeted” by antibodies. Each antibody (immune response) is specifically produced by the immune system to match an antigen after cells in the immune system come into contact with it; this allows a precise identification/matching of the antigen and the initiation of a tailored response. The antibody is said to “match” the antigen in the sense that it can bind to it due to an adaptation performed to a region of the antibody; because of this, many different antibodies are produced, each with specificity to bind a different antigen while sharing the same basic structure. In most cases, an adapted antibody can only react to and bind one specific antigen; in some instances, however, antibodies may cross-react to and bind more than one 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 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.