Immune responses result from the activation of an elaborate network of interacting cells occurring in specific microenvironments. Attention in recent years has been focussed intensively on a number of issues. Included among these are: the developmental pathways of the two major types of lymphocytes, T and B cells; the nature of their specific receptors for antigen and of their various accessory surface molecules; the subsequent fate of the cells following encounter with antigen presented as such or on a variety of cells; and the function of the effector molecules produced after successful activation by antigen. The present volume is concerned with some of these issues, and also with their potential application to the control of autoimmune phenomena and parasitic infestations.