By Chang - Yuil Kang and Heinz Kohler

The surface of the immunoglobulin structure plays a multifunctional role within the immune network. The variable structures of antibodies express a large repertoire of idiotypic specificities (idiotopes) and are also responsible for the vast diversity of different binding sites (paratopes). Crystallographic analyses of antibodies have identified V regions that form a binding site or pocket for binding with antigen (1). The sequence regions that participate in the antigenbinding site have been correlated closely with the most variable sequence regions of the domain, and were described as complementarity-determining regions (CDR) (2). For communication with antibodies and receptors of immunocompetent cells, the Ig surface has developed another variable repertoire that expresses the diversity of idiotopes. Very little is known about the structural correlates of idiotypic determinants except that they must be accessible for antibodies to bind to. This implies that idiotopes are autoantigens, as they are recognized as self antigens by the immune system (3). The size of the idiotope repertoire produces the statistical necessity that the shapes of other structures are mimicked. This stereochemical mimicry generates what immunologists call internal antigens or homobodies (4, 5). Since the expression of idiotopes is, at least in part, made by an independent topographical area of the Ig surface (6), it should be possible for the same Ig structure to possess idiotope and paratope of complementary configuration. Mimicry of Ig determinants by idiotopes has been described recently by Bona and colleagues (7), and by Chen and coworkers (8). The term epibody was developed to signify the presence of an antibody recognizing both an idiotope and an epitope on the original antigen. In their example, the antigenic determinant mimicked by an idiotypic determinant may be the target for an antiidiotypic antibody (epibody). Herein, we describe an antibody that mimicks the antigen for its own paratope site. The specificity of the antibody is for phosphorylcholine (PC), and PC is mimicked and expressed as idiotope by the same antibody. Coexpression by the same molecule of paratopes and idiotopes that possess complementary structures may have profound implications for the understanding of the network concept (8), the role in autoimmunity (9), and the design of idiotype vaccines (10). This work was supported in part by grants awarded to H. Kohler from the American Cancer Society (IM-405), the National Institute on Aging, Department of Health and Human Services (AG04180), and the Council for Tobacco Research-USA, Inc. (1565).