Dendritic cells are a system of bone marrow-derived antigen-presenting cells specialized for interaction with T lymphocytes and essential for initiating primary T cell immune responses. Recent investigation indicates that dendritic cells are of diverse origin, with at least two types of myeloid precursors and a lymphoid precursor implicated in their generation. Mature dendritic cell subtypes, while sharing the capacity to activate T cells, show additional functional specialization. Some dendritic cells are equipped with additional mechanisms to regulate the response of the T cells they activate, while others are able to interact with B cells and modify B cell responses.