<P> 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 . </P> <P> 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 . </P>

Any substance that can provoke an immune response is an