Immune reactions and mental disorders

Is the immune response altered in mental illness? Conversely, does an immune response affect mental function? Are any observed changes (both mental and immunological) the result of aetiological relationships? The answers to these questions are far from straightforward. The field is complex and evidence from immunological, psychiatric, psychological and neuroendocrinological studies is still scarce and fragmentary. On the other hand, studies aimed at finding a linear causal relationship are perhaps inappropriate for biological interactions between such multivariate systems as the immune system and the central nervous system (CNS) (Bennette, 1969). There are, however, good reasons to draw a parallel between the two systems and to postulate links between them. Both serve functions of adaptation and defence, and both intervene in the relationship between the individual and the environment. The CNS, the site of emotions, personality and ego defences, shares the same responsibilities as the immune system which is that of discriminating between self and non-self and protecting the integrity of the self against intrusions of non-self. Both systems have flexibility and a diversity of responses that incorporate principles of recognition, learning, memory and transmission of information. The immune response can be modified by the state of mind. This is shown by studies with hypnosis (Black et al. 1963), meditation (Smith et al. 1985) and conditioning (Ghanta et al. 1985). Psychological experiments in animals have shown that early experiences like stress and separation can influence the immune response later in life (Solomon et al. 1968; Laudenslager et al. 1983). Also, direct intervention in the CNS modifies immune response. For example, experimental hypothalamic lesions in animals decrease lethal anaphylaxis (Stein et al. 1976). A link in the converse direction is shown by studies with implanted electrodes which have demonstrated direct regulation of the hypothalamus by an immune response (Besedovsky et al. 1983). From an accumulation of data such as this, a feedback loop between the hypothalamus and the immune system has been suggested (Solomon & Amkraut, 1984). The neuroendocrine system, also involved in the process of adaptation, has been proposed as the link and common path between the CNS and the immune system. The interaction of these three systems has been supported by studies on stress (for review, see Farrant & Perez, 1987) and by the close and often interdependent relationship between the CNS, the hypothalamus-pituitary-adrenal axis and the immune system. The potential for links between the CNS and the immune system is further suggested by the presence of receptors for neuroand other hormones on lymphocytes. Many of these hormones are able to modify lymphocyte function (see Farrant & Perez, 1987). Bohus (1984) has proposed that an old model be extended to the immune system. 'Neural messages' are translated into 'hormonal messages' which then affect various target organs, including the adrenal gland and hence the lymphocytes of the immune system. Information from target organs can feedback to the CNS, closing the cycle of homeostatic adaptive interactions. What are the implications of these interactions for mental illness? The best studied and most clear evidence comes from investigations done on stress. Stress is known to have a direct effect on behaviour, neurohormone release and immune response (see Farrant & Perez, 1987). It is important, however, to avoid a simplistic notion of stress as being the 'cause' of disease, but look rather at its effect in influencing the expression of disease in the interaction between personality and