ABSTRACT
An infectious agent, whether a bacterium, fungus, virus, or parasite, contains an abundance of substances capable of inducing an immune response. These are called immunogens or antigens. Specifically, an immunogen is a substance capable of stimulating B cell, T cell, or both limbs of the immune response. An antigen is a substance that reacts with the products of an immune response stimulated by a specific immunogen, including both antibodies and/or T lymphocyte receptors. The “traditional” definition of antigen more correctly refers to an immunogen. A complete antigen is one that both induces an immune response and reacts with the products of it, whereas an incomplete antigen or hapten is unable to induce an immune response alone but is able to react with its products, e.g., antibodies. Haptens could be rendered immunogenic by covalently linking them to a carrier molecule.
