Accessory cells unrelated to mononuclear phagocytes and not of bone marrow origin

The induction and regulation of immune responses require the participation of cells which maintain lymphocyte viability, promote necessary cellular interactions, secrete immunoregulatory molecules and present antigen. These are accessory cells, and their function is not antigen-specific. Classically, cells of the mononuclear phagocyte lineage have been considered to be the major accessory cell population in most immune responses. More recently, other cell types, such as Langerhans cells of the skin and dendritic cells of the lymphoid organs, have also been shown to be effective. This article reviews the evidence that additional cell types, such as endothelial cells or fibroblasts, serve an accessory role in immune responses.