In the human body, what word describes any foreign substance which induces an immune response?

In immunology, an antigen is a molecule capable of inducing an immune response in the host organism. Sometimes antigens are part of the host itself in an autoimmune disease.

Antigens are "targeted" by antibodies. Each antibody 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 or 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.