Dendritic cells (DCs) are a heterogeneous population of bone marrow-derived antigen-presenting cells (APCs) that populate various lymphoid and nonlymphoid tissues, including the skin (Langerhans cells), lymph nodes (interdigitating and follicular DCs), spleen, and thymus. DCs are considered professional APCs, with a remarkable ability for initiating T-cell activation and the consequent immune response, but also for inducing tolerance (1–3). The regulation of these two distinct functions is not completely understood to date. Extensive recent studies have improved our understanding of DC development, differentiation, activation, and function. DCs exist as distinct subsets that differ in their lineage affiliation, surface molecule expression, and biological function. Each of these types of DCs can present antigens in either an immunogenic or tolerogenic manner according to their maturation state and antigen capture properties. Hence, DCs either induce the appropriate immune response to pathogens or prevent autoimmune reactivity, and could thus be useful for manipulating the immune system by cellular immunotherapy.