Health Psychology. Michael Murray
of hormones and receptors. Glucocorticoids, such as corticosterone and cortisol, regulate inflammation levels and have effects both on the innate and adaptive immune responses. Additionally, vitamin D affects immune cell responses by enhancing antigen presentation. Moreover, sex hormones affect the immune response in numerous ways. The effects of hormones on microbiota are summarized in Figure 2.11.
Figure 2.11 Host effects on gut microbiota
A variety of host factors (such as diet, exercise, mood, general health state, stress and gender) lead to alterations in hormonal levels, which in turn lead to a variety of effects on the microbiota (including growth, virulence and resistance)
Source: Reproduced with permission from Neuman et al. (2015)
The Immune System
The immune system (IS) is a network of cells, tissues and organs that protects the body against disease or other potentially damaging foreign bodies. When properly functioning, the IS identifies and attacks a variety of threats using billions of diverse antibodies, including viruses, bacteria and parasites, while distinguishing them from the body’s own healthy tissue. For each type of invader the body needs a distinct antibody. Antibodies are made by B cells using a combination of 20,000 genes and an enzyme called ‘RAG’, which is a DNA shuffler. This enables the immune system to create a vast diversity of antibodies and respond to diseases it has never encountered before.
The IS is composed of two parts: the innate IS and the adaptive IS. Both change as people get older. The main features of the IS are illustrated in Figure 2.12.
Figure 2.12 Organs of the immune system
Our innate IS is made up of barriers and cells that keep harmful germs from entering the body. These include our skin, the cough reflex, mucous membranes and stomach acid. If germs are able to pass through these physical barriers, they encounter a second line of innate defence, composed of specialized cells that alert the body to the impending danger.
The IS changes over the lifespan. Newborn babies have an immature IS. Immunological competence is gained after birth partly as a result of maturation factors present in breast milk and partly as a result of exposure to antigens from food and environmental micro-organisms. Early encounters with antigens help the development of tolerance, and a breakdown in ‘immune education’ can lead to disease (Calder, 2013). At the end of the life-cycle, older people experience progressive dysregulation of the IS, leading to decreased acquired immunity and a greater susceptibility to infection. Innate immunity appears to be less affected by ageing than acquired immunity.
A healthy, young person’s body produces numerous T cells and is able to fight off infections and build a storehouse of memory T cells. With age, people produce fewer naïve T cells, which makes them less able to combat new health threats. This also makes older people less responsive to vaccines because vaccines generally require naïve T cells to produce a protective immune response (except in the case of the shingles vaccine). Negative, age-related changes in our innate and adaptive immune systems are known as immunosenescence. A lifetime of stress on our bodies is thought to contribute to immunosenescence. Radiation, chemical exposure and exposure to certain diseases can also speed up the deterioration of the IS.
The adaptive IS is more complex than the innate IS and includes the thymus, spleen, tonsils, bone marrow, circulatory system and lymphatic system. These different parts of the body work together to produce, store and transport specific types of cells and substances to combat health threats. T cells, a type of white blood cell (called lymphocytes), attack infected or damaged cells directly or produce powerful chemicals that mobilize an army of other IS substances and cells. Before a T cell is programmed to recognize a specific harmful germ, it is in a ‘naïve’ state. After a T cell is assigned to fight off a particular infection, it becomes a ‘memory’ cell. Because these cells remember how to resist a specific germ, they help to fight a second round of infection faster and more effectively. Memory T cells remain in our systems for many decades.
An important part of our adaptive IS is the lymphatic system, consisting of bone marrow, spleen, thymus and lymph nodes. Bone marrow produces white blood cells, or leukocytes. The spleen is the largest lymphatic organ in the body and contains white blood cells that fight infection or disease. The thymus is where T cells mature. T cells help destroy infected or cancerous cells. Lymph nodes produce and store cells that fight infection and disease. Lymphocytes and leukocytes are small white blood cells that play a large role in defending the body against disease. The two types of lymphocyte are B cells, which make antibodies that attack bacteria and toxins, and T cells, which help destroy infected or cancerous cells.
Inflammation
Inflammation is a critical defence response in our innate IS wherein white blood cells protect us from infection by foreign organisms, bacteria and viruses. Inflammation occurs following infection or tissue damage when a rapid and complex series of reactions takes place to prevent tissue damage, isolate and destroy the infective organism, conserve and protect some micronutrients and activate the repair processes to restore normal function (Thurnham, 2014). Inflammation is a homoeostatic process that is only intended to last a few days but, if it is continued indefinitely, there is a poor prognosis in many conditions. Inflammatory responses take precedence over normal body metabolism with the objective of restoring normality as quickly as possible.
In a young person, bouts of inflammation are vital for fighting off disease. As people age, they tend to have mild, chronic inflammation, which is associated with an increased risk for heart disease, arthritis, frailty, Type 2 diabetes, physical disability and dementia. Whether inflammation leads to disease, disease leads to inflammation, or whether both scenarios are true currently remains uncertain. Centenarians and other people who have grown old in relatively good health generally have less inflammation and more efficient recovery from infection and inflammation when compared to people who are unhealthy or have average health. Acute inflammation on a timescale of seconds to days allows the host to heal and protect damaged tissue from disease. Chronic inflammation lasting weeks or months is linked to many pathologies and age-related diseases, including sleep apnea, insomnia, neurodegeneration, Alzheimer’s disease, atherosclerosis, cancer, kidney and lung diseases, metabolic syndrome and Type 2 diabetes mellitus.
Disorders of the IS can result in autoimmune diseases, inflammatory diseases and cancer. When the IS is less active than normal, which is called immunodeficiency, recurring and life-threatening infections can occur. Immunodeficiency can result from a genetic disease, acquired conditions such as HIV infection (see Chapter 22), or the use of an immunosuppressive medication.
The opposite situation of autoimmunity results from a hyperactive IS attacking normal tissues as if they were foreign organisms. Common autoimmune diseases include Hashimoto’s thyroiditis, rheumatoid arthritis, Type 1 diabetes mellitus (see Chapter 23) and systemic lupus erythematosus. Autoimmune diseases are chronic conditions with no cure. Treatment requires controlling the disease process to decrease the symptoms, especially during flare-ups. The following actions can alleviate symptoms of autoimmune disease: a balanced and healthy diet, regular exercise, plenty of rest, vitamin supplements (especially A and D), a decrease of stress, and the avoidance of any known triggers of flare-ups. Sound familiar? Hippocrates knew about them and your doctor’s waiting room has a poster.
Circadian rhythm of the immune system
Circadian variation occurs in immunocompetent cells and cytokines as an anticipatory process for the preservation of body homeostasis and defence (Cermakian et al., 2013). Cytokines are small protein cells