Abstract: Until recently, mast cells have been viewed primarily as harmful because of their key role as effector cells of allergic and potentially lethal anaphylactic reactions. Their contribution to human health appeared instead to be limited to the elimination of parasites. There is, however, growing evidence for additional beneficial functions of mast cells, particularly regarding the initiation of acquired immune reactions. Thus, mast cells can phagocytize diverse particles, take up antigens, and express a number of receptors, particularly MHC class I and II antigens, ICAM-1 and -3, CD43, CD80, CD86 and CD40L which allow them to interact with T and B lymphocytes. They can also secrete numerous cytokines that induce and enhance recruitment and functions of lymphocytes. Finally, there is good evidence that mast cells present e.g. pollen and bacterial antigens, respond to bacterial superantigens, but fail to react to endogenously produced antigens or superantigens. Mast cells can also activate B cells directly to produce IgE, but this activity and the ability to produce IL-4 or IL-13 is restricted primarily to basophil leukocytes and mucosal mast cells. Finally, recent evidence attributes a pivotal role to the cells in natural immunity to bacteria. There is also emerging evidence that mast cells can downmodulate the immune response. While these data require further clarification, the basic ability of mast cells to initiate innate and acquired immune reactions can no longer be questioned.