Role of macrophages in the expression of immune responses.

: Cells of the mononuclear phagocyte system have a crucial role as affector and effector cells in the body's defense against foreign cells and microorganisms. Macrophages function as the first line of defense via phagocytosis or opsonic phagocytosis as early as the promonocytic stage of their development. Macrophages act as affector cells via antigen presentation to lymphocytes, and they participate in the activation of T and B lymphocytes through the secretion of lymphostimulatory substances (monokines). In the cycle of reciprocal interactions, macrophages are themselves being activated via the secretory products of the lymphocytes--the lymphokines. Activated macrophages are endowed with effector functions exerted by their tumoricidal, microbicidal, and suppressor activities. Undoubtedly, additional research will enhance the importance and application of this unique cell type with multiple functions.