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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.