ABSTRACT
For more than a century it has been evident that the various tissues of an organism are structurally different at both the morphological and molecular levels. In terms of immunobiology, this is reflected in the fact that tissues are antigenically distinct-that is, each tissue possesses unique macromolecules capable of stimulating a specific immune response within a foreign host. This fact has certainly been obvious to the biochemist, as each tissue has a relatively unique enzymatic content and thus would be expected to be antigenically distinct. An antigen may be defined as a substance capable of eliciting an immune response when the immune system of the organism is exposed to the antigen. An antibody is a circulating globulin specifically reactive with the antigen responsible for its production or with a comparable (cross-reactive) antigenic species. Antigens that are relatively specific to the tissue in which they are found have been termed tissue-specific antigens (Milgrom, 1966) or, in another context, differentiation antigens. A differentiation antigen is operationally defined as a determinant, detected in immunological tests (usually specific antibody-containing sera), that is limited in its pattern of expression to specific tissues (cf. Old et al., 1962; Old and Stockert, 1977). Lymphocyte populations express particular differentiation antigens on their surfaces. When a number of monoclonal antibodies (Chapter 14) recognize or react with the same differentiation antigen, these antibodies define clusters of differentiation, or CD. The nomenclature for some of these CD antigens is considered below.
