B cells used to be considered as a homogeneous population of cells destined to produce antibodies of increasing affinity and to maintain an immunological memory. In recent years, it has been determined that B cells can be subdivided into different subsets characterized by distinct morphologic, phenotypic, and functional features. Presenting results of research work on the definition of B cell subset populations, this book explains the basic mechanisms that control B cell activation, stimulation and regulation. Articles include studies on both normal and malignant B cells and describe the mechanisms underlying T-B cell interactions during the immune response. The most important advances in the field of immunodeficiency are also reported. This volume will be essential not only for basic and clinical immunologists, but also for hematologists, pathologists and rheumatologists with a special interest in the pathogenesis of lymphoproliferative or autoimmune disorders.