An extraordinary development of the knowledge, concepts and biomedical applications has occurred during the past two decades in the biological sciences, including the Reticuloendothelial Sys- tem (RES). For example, it is now widely recognized that distinct classes of cells are involved in the recognition of "exogenous" invaders of the body such as microorg nisms and other foreign an- tigens, as well as of "endogenous" parasites represented by trans- formed neoplastic cells and altered "self" antigens. Prominent among cell populations involved in such recognition of antigens and subsequent...