Health Psychology. Michael Murray
The IS shows reliable daily variations, for example immunocompetent cell counts and cytokine levels vary according to the time of day and the sleep–wake cycle. Different immune cell types, such as macrophages, natural killer (or NK) cells, and lymphocytes, all contain circadian molecular clockwork.
The biological clocks of immune cells and lymphoid organs, together with the central pacemaker of the suprachiasmatic nuclei via humoural and neural pathways, regulate the IS, including its response to signals and their effector functions. There is a diurnal variation in the response to immune challenges (e.g., a bacterial injection) and circadian control of allergic reactions. The circadian–immune connection is bidirectional, and immune challenges and immune mediators (e.g., cytokines) affect circadian rhythms at the molecular, cellular and behavioural levels. Cross-talk between the circadian and immune systems has implications for disease, as shown by the higher incidence of cancer and the exacerbation of autoimmune symptoms upon circadian disruption (Cermakian et al., 2013).
Sleep and rest
Sleep and rest are necessary for a properly functioning IS. Feedback loops involving cytokines in response to infection participate in the regulation of non-rapid eye movement sleep. In sleep deprivation, active immunizations may have a diminished effect, resulting in lower antibody production and a lower immune response than for well-rested individuals. Proteins such as NFIL3, which are closely intertwined with both T-cell differentiation and our circadian rhythms, are affected by disturbances of natural light and dark cycles through instances of sleep deprivation, travelling across time zones or shift work. Such disruptions on a regular and frequent basis can lead to an increase in chronic conditions such as heart disease, chronic pain and asthma.
Sleep, sleep loss and disrupted sleep are strongly linked to acute and chronic inflammation (Opp and Krueger, 2015). People suffering from sleep deprivation demonstrate changes in circulating pro-inflammatory and anti-inflammatory cytokines, soluble receptors, inflammatory signalling pathways and innate immunity. Circadian misalignment also induces inflammation, which has ramifications for shift workers. Shift work is a risk factor for inflammatory diseases, including cancer and diabetes. In addition to the negative consequences of sleep deprivation, sleep and the circadian system have regulatory effects on immunological functions in both innate and adaptive immunity.
Chronic diseases that are associated with suboptimal sleep are inflammatory diseases. Chronic insufficient sleep is a risk factor in part because of the inflammatory state that results from sleep disruption. Inflammation, defined by elevated local and systemic cytokines and other pro-inflammatory mediators, occurs in response to many stimuli, including pathogen exposure, cellular damage, irritants, cellular dysregulation and waking activity.
Nutrition and diet
Consumption of high-fat (e.g., ice cream – 11g fats per 100g serving) and high-sugar (e.g., tomato ketchup – 23.6g sugar per 100g serving) foods is associated with conditions such as diabetes and obesity, which can affect immune function. Moderate malnutrition, as well as certain specific trace mineral and nutrient deficiencies, also can compromise the immune response. Undernutrition-associated impairment of immune function can be due to insufficient intake of energy and macronutrients and/or due to deficiencies in specific micronutrients. Foods rich in certain fatty acids may foster a healthy IS. Foetal undernourishment is especially risky as it can cause a lifelong impairment of the IS. A variety of micronutrients have been implicated in improving the immune response, especially Vitamin A, Vitamin D, Vitamin E, zinc, iron and selenium.
Psychoneuroimmunology
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.
Homeostasis
Homeostasis refers to the principle by which the internal environment of the body is kept in a state of equilibrium by a multitude of fine adjustments at a hierarchy of levels ranging from the molecular level to the level of the organism as a whole. Claude Bernard (1865) first described what he called the ‘internal milieu’ and showed that this internal environment was ordinarily maintained within fixed limits. Walter Cannon (1932), in The Wisdom of the Body, coined the term ‘homeostasis’ for the coordinated physiological processes by which an organism maintains an internal steady state. Both Bernard and Cannon focused almost entirely on physiological homeostasis. Curt Richter (1942) expanded the idea of the protection of the internal milieu to include behavioural or ‘total organism regulators’. From this viewpoint, behaviour lies on a continuum with physiological events. Richter combined the perspective of Bernard with that of Cannon and he added behavioural regulation.
Behaviour, for Richter, was broadly conceived to include all aspects of identification, acquisition and ingestion of the substances needed to maintain the internal environment. The current theory extends the homeostasis concept one step further in suggesting that not only feeding, but all human behaviour, follows the principle of homeostasis. Psychological homeostasis is best explained in two stages, starting with the classic version in Physiology, followed by the new version extended to Psychology. Physiological homeostasis is illustrated in Figure 2.13.
There are five critical components that