Dendritic cells are a system of antigen presenting cells that function to initiate several immune responses such as the sensitization ofMHC-restric ted T cells, the rejection of organ transplants, and the formation of T dependent antibodies. Dendritic cells are found in many nonlymphoid tissues but can migrate via the afferent lymph or the blood stream to the T· dependent areas of lymphoid organs. In skin, the immunostimulatory function of dendritic cells is enhanced by cytokines, especially GM-CSF. After foreign proteins are administered in situ, dendritic cells are a prin cipal reservoir of immunogen. In vitro studies indicate that dendritic cells only process proteins for a short period of time, when the rate of synthesis ofMHC products and content of acidic endocytic vesicles are high. Antigen processing is selectively dampened after a day in culture, but the capacity to stimulate responses to surface bound peptides and mitogens remains strong. Dendritic cells are motile, and efficiently cluster and activate T cells that are specific for stimuli on the cell surface. High levels of MHC class-I and-II products and several adhesins, such as ICAM·! and LFA· 3, likely contribute to these functions. Therefore dendritic cells are spe cialized to mediate several physiologic components of immunogenicity