As pointed out in Chapter 1, the nervous, endocrine and immune systems interact in many ways. Damage to the brain or changes in neurotransmitter and neurohormone release alter immune responses and the chemical messengers released by the cells of the immune system can alter the activity of the nervous and endocrine systems (Smith and Blalock, 1986; Kordon and Bihoreau, 1989). This chapter begins with an overview of the cells of the immune system and their chemical messengers, the cytokines, and then discusses the immune functions of the thymus gland and its hormones. The functions of the cytokines in the immune response to antigens and in the development of blood cells are then summarized and the neuromodulatory effects of cytokines on the brain and neuroendocrine system are examined. This is followed by a discussion of the neural and endocrine regulation of the immune system and the hypothalamic integration of neural, endocrine and immune systems.

THE CELLS OF THE IMMUNE SYSTEM

The immune system consists of a number of different cell types, including the monocytes and macrophages, T lymphocytes (T cells), B lymphocytes (B cells), granulocytes and natural killer (NK) cells. The role of the immune system is to help maintain homeostasis in the body and its best known function is the protection of the body from foreign invaders such as bacteria and viruses and from abnormal cellular development as occurs in tumor cells. To do this, the immune system must be able to discriminate foreign (non-self) cells from the body's own cells (self). Almost all substances have regions called antigenic determinants or ‘epitopes’ which can stimulate an immune response.