or complement, or a results of the function of natural killer (NK) cells or phagocytes or complement, or a resu or barriers such as mucosal surfaces. The natural immune system is able to attack a substance to which it has had no previous exposure; subsequent exposure to that stimulus does not increase the response. On the other hand, the specific immune response recognizes a specific feature (antigen) of the foreign substance or cell and generates an immunologic memory, which amplifies responses during subsequent exposures. The specific immune system can recruit portions of the innate system t function simultaneously. Lymphocytes are the primary effectors of the specific im mune response. These cells "direct" much of the immunologic activity by the pro duction of cytokines of many types. Cytokines are soluble protein molecules that function as messengers to other leukocytes. They are produced and secreted primarily by monocytes and their main targets are lymphocytes and other monocytes. Some of the important cytokines in clude interleukins interferons, and tumor necrosis factors ITNFs). There are at least 5 distinct forms of interleukins, 3 inteferons, and 2 tumor necrosis factors