The ac ti vati on of Iymphocytes by physiologie ligands has become a central area of study for immunologists. Furthermore, Iymphocytes offer many advantages for the study of general aspects of receptor-mediated cellular activation. For these reasons, research on the cellular and molecular aspects of the activation of Iymphocytes by antigens, antigen analogues, Iymphokines and other growth factors has expanded enormously in the last several years...