Number and function of leukocytes is regulated by the autonomic nervous system

Macrophages, constituting a host defense system, are the most fundamental leukocytes which protect the human body from bacteria and antigens. In the course of phylogenetic development, multicellular organisms acquired granulocytes and lymphocytes from proto-macrophages. Granulocytes are beneficial for eliminating bacteria by their phagocytosis, whereas lymphocytes are beneficial for eliminating small antigens by their immune functions. The distribution of granulocytes and lymphocytes (e.g., 60%:35% in the peripheral blood of adult humans) is known to be influenced by the size of microbes that invade the human body. In this review, it is also revealed that the distribution of these leukocytes is regulated by the autonomic nervous system. This is due to the existence of adrenergic receptors on granulocytes and the existence of cholinergic receptors on lymphocytes. For the most part, the variety of leukocytes induced by the autonomic nervous system appears to be desirable for defense of the host. However, if the autonomic nervous system deviates too much in one direction, over activation of granulocytes or lymphocytes appears, which results in certain diseases. Without introduction of the concept of 'immunomodulation by the autonomic nervous system,' the mechanisms underlying the onset of many tissue destructive diseases and allergic diseases cannot be properly understood.