Lymphocytes have an essential role in antibody formation and cell-mediated immunity, the two major classes of immune response. The latter class includes delayedtype hypersensitivity, the homograft reaction and certain autoimmune responses. Primary antibody responses and the ability to develop cellular immunity are impaired in animals depleted of lymphoctyes. When the lymphocyte deficiency is corrected either spontaneously or by injections of lymphoctyes from normal animals, immunological responsiveness is restored (Gowans and McGregor, '65). The results implicate lymphoctyes in immunity, but provide little information about the role they play.
Efforts to define the immunological activity of lymphocytes have been hampered by the structural and functional diversity existing among cells of this type. Individual lymphocytes vary greatly in size and structure. Furthermore, cells which fall into the same morphological category differ with respect lo life-span (Everett, Cafβ¬rey and Rieke, '64; Robinson, Brecher, Lourie and Haley, '65), and possibly with respect to origin and developmental capability.
Recent studies of the life cycle of lymphocytes provide new insight into their immunological performance. It is now clear that the evolution of the antibody response depends upon the circulation of lymphocytes through lymph nodes and spleen. Hall and Morris ('64 ) observed that a dose of 2000r applied to a popliteal lymph node in sheep causes only a moderate decrease in cell output from the node into efferent lymph and no diminution in the antibody response to a regional injection of Salmonella typhi. Apparently the antibody response in locally irradiated lymph nodes is enacted by unirradiated cells (presumably small lymphocytes) which move into