Signaling through antigen receptor initiates a complex series of events resulting in the activation of genes that regulate the development, proliferation and differentiation of lymphocytes. During the past few years, rapid progress has been made in understanding the molecular basis of signaling pathways mediated by antigen and cytokine receptors. These pathways involve protein tyrosine kinases which are coupled to downstream regulatory molecules, including small guanine nucleotide binding proteins (e. g. p21'OS), serine threonine kinases (e. g. , members of the ERK family), and a large group of transcription factors. More recently, there have been breakthroughs in elucidating the genetic defects underlying three X-linked primary immunodeficiency diseases in humans. This volume surveys aspects of these rapidly developing areas of research. The book is divided into 5 different sections. Section I deals with signaling pathways in B lymphocytes. It includes a contemporary assessment of B cell antigen receptor structures, and discussion of the role of Ig-a/lg-B polypeptides in linking the antigen receptor to intracellular signal transduction pathways. The role of accessory molecules in the regulation of signaling by the B cell antigen receptor is also considered. Section II adopts a similar approach to the analysis of the antigen receptor on T lymphocytes. The importance of specialized signaling motifs in the CD3 polypeptides, mechanisms whereby these motifs may interact with the lymphocyte-specific protein tyrosine kinases, and the downstream consequences of these interactions are reviewed. In addition, the role of antigen-induced apoptosis in the generation of immunological tolerance is discussed.