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The study of the interactions between psychological, neurological and immunological processes constitutes the field of ‘psychoneuroimmunology’, but ‘PNI’ will do just fine. As we have already seen, the immune system and CNS maintain extensive communications. The brain modulates the IS by hardwiring sympathetic and parasympathetic nerves to lymphoid organs. The IS modulates brain activity, including sleep and body temperature. Based on close functional and anatomical links, the immune and nervous systems act in a highly reciprocal manner. From fever to stress, the influence of one system on the other has evolved in an intricate manner to help sense danger and to mount an appropriate adaptive response. Research over recent decades suggests that these brain-to-immune interactions are highly modulated by psychological factors that influence immunity and IS-mediated disease.

The brain and the IS are involved in functionally relevant cross-talk, with homeostasis being the main function. The CNS is without lymphatic drainage and so lacks the immune surveillance available for the rest of the body. However, there are mechanisms to exclude the potentially destructive lymphoid cells from the brain, spinal cord and peripheral nerves, ranging from small molecules, such as nitric oxide, to large proteins, including cytokines and growth factors, which tie the two systems together.

Recent studies in PNI are indicating many empirical links between the psychological, endocrinological and immunological systems. PNI research remains at a relatively early stage of development, with many publications having an empirical rather than a theoretical focus.

In a randomized controlled experiment, people who performed kind acts for others showed favourable changes in immune cell gene expression profiles (Nelson-Coffey et al., 2017). High sensitivity C-reactive protein (hs-CRP) has emerged as a marker of inflammation in atherosclerotic vascular disease.

Tayefi et al. (2017) measured symptoms of depression and anxiety and serum hs-CRP levels in 9,759 participants (40% males and 60% females) aged 35–65 years in north-eastern Iran. They found that depression and anxiety are associated with serum levels of hs-CRP, higher BMI in women, and smoking in men.

Blair et al. (2017) analysed a prospective longitudinal sample of 1,292 infants in predominantly low-income and rural communities from infancy through to age 60 months. For children with relatively low cortisol levels between the ages of 7, 15, 24 and 48 months, those illustrating moderate fluctuations in their cortisol levels over this span tended to show subsequently better executive function (EF) performance at 60 months than did children with either highly stable or highly variable temporal profiles. This curvilinear function did not extend to children whose cortisol levels were high on average, who tended to show lower EF performance, irrespective of the stability of their cortisol levels over time.

PNI research suffers from the same ailments as most other areas of health psychology: it is underpowered with small sample sizes, cross-sectional designs and lack of replication. It is a field that has been hyped but is yet to reach its full potential.