- Occupation of cell surface receptors on lymphocytes by appropriate antigens and/or growth and differentiation factors initiate activation steps essential for acquisition of differentiated functions and clonal expansion. The diversity of lymphocyte antigen-specific receptors and selective expression of binding sites for distinct growth and differentiation factors form the basis for the specificity of the immune response. In contrast to the diversity of receptor-associated recognition systems, transduction of signals to lymphocyte nuclei seems to occur via a limited number of biochemical pathways. One ubiquitous mechanism of signal transduction involves the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP 2 ) and formation of two second messengers, diacylglycerol (DG) and inositol 1,4,5-trisphosphate (IP 3 ), which trigger numerous physiological responses in T and B lymphocytes. Lymphocytes which are triggered by different or even identical ligands can use a single signal transduction pathway but respond with any of several genetically predetermined effector functions. Ligands and cell surface receptors that participate in lymphocyte activation, and the intracellular events accompanying this activation step were the topic of a recent meeting.