Recent evidence demonstrates that the antigen receptor complexes of T and B lymphocytes are very similar in general architecture and primary and secondary structure of component polypeptides, and that they use common mechanisms for transmembrane signal transduction. Most importantly, multiple subunits of each receptor (Ig-alpha, Ig-beta and Ig-gamma, CD3 gamma, CD3 delta and CD3 epsilon, and T-cell receptor zeta and eta) possess a motif of approximately 26 amino-acids, denoted ARH1, which appears to carry sufficient structural information for receptor-mediated lymphocyte activation.