The Immune System

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What is the immune system?

The immune system is a complex biological system, consisting of many different types of cells, tissues and organs, each with their own specific function. The immune system’s major functions are to recognise and neutralize harmful substances and infective organisms (like bacteria, viruses and parasites), heal damaged tissue and remove cells of our own body that are not functioning correctly, such as cancer cells. The immune system gets stronger as we approach adulthood, but again gets weaker in old age. This is why children and the elderly are more susceptible to illness.

The most important ability of the immune system is to be able to distinguish between self and non-self, or in other words, normal and abnormal. If it is not able to do this properly, it will either allow infectious organisms to invade, or begin attacking your own body. The latter is known as autoimmune disease. Early on, the body learns to recognise specific proteins that are on the surface of the body’s cells as ‘self’. As we are exposed to infectious organisms, the body learns to recognise proteins on the surface of the invaders cells, known as antigens, which signal that it should attack them.

What are the different types of immunity?

There are three types of immunity in humans called innate, adaptive, and passive immunity.

What is innate immunity?

Innate immunity consists of the parts of the immune system that you are born with and that do not change significantly throughout life. It includes defensive structures such as the skin and mucous membranes, as well as a number of white blood cell types such as Natural Killer Cells. These are general defences, which means that they don’t recognise specific invaders, but rather that you have been infected and attempt to attack all invaders in a similar way. The innate immune system is your body’s first line of defence against new threats. While it can often deal with infections before they become an issue, invaders are sometimes able to overcome it. This is where adaptive immunity comes in.

What is adaptive immunity?

Adaptive, or acquired, immunity is the part of the immune system that changes over time, learning to recognise new invaders and mount a response to them. This is known as immunological memory.

This memory is why vaccines can confer immunity, as well as why we do not get certain diseases (like chickenpox) twice. This is also the part of the immune system which takes the most time to develop. In children, the adaptive immune system is largely untrained, which is why they get sick so much more often. By adulthood, we have come into contact with many more pathogens and therefore are able to stay healthier longer.

The adaptive immune system relies on two special types of white blood cell, called lymphocytes. These are B-cells and T-cells. B-cells do not directly attack invaders, but are more involved in remembering what they look like and ensuring T-cells (which do attack invaders) and other immune cells can recognise them. One way they do this is through production of antibodies. Antibodies are protein molecules which circulate through the body and bind to the surfaces of invading organisms. Through binding they coat the invader (making it harder for it to move around or get inside cells) and signal to T-cells to attack.

What is passive immunity?

Passive immunity is the passing of resistance to a disease from one person to another. An important role of passive immunity is to protect new-born infants from infection. Various antibodies are transferred from mother to child in the womb and through breast milk. The infant therefore temporarily receives the mother’s highly developed immunity while its own immune system begins to develop.

Passive immunity is short-lived (from days to several months), and the immunity to a wide range of organisms that an infant has while breast feeding is gradually lost as it is weaned. Toddlers and older children will therefore not be resistant to the complete range of organisms their parents are. As such, focus on supporting their growing immune system is paramount to prevent illness and develop their adaptive immune system.

What happens to the immune system as I age?

As with many other biological functions, the immune system becomes less effective as you age. This includes its ability to recognise and ward off pathogens, prevent the proliferation of cells of our own body that cause disease, as well as its ability to recognise self from non-self. As we age, we therefore become more likely to develop a number of diseases, including autoimmune disorders, and become more susceptible to invading organisms.

It is therefore important to focus on immune health as you age. Some micronutrient deficiencies are also more prevalent in the elderly, and these have been linked to a decline in the immune system.

How can you strengthen your immune system?

The immune system is complex, relying on the correct function of a wide variety of cells, tissues and organs. Optimal immune function therefore relies on maintaining a healthy lifestyle.

This includes following a healthy, well-balanced diet, getting an adequate amount of sleep, regular exercise, reducing stress, and avoiding toxic substances.

Deficiencies in zinc, selenium, iron, copper, folic acid and vitamins A, B6, C, D and E have been associated with poor immunity and higher rates of disease. Ensuring you are receiving optimal amounts of these nutrients is therefore essential. These nutrients can be obtained from a healthy diet, centred on plant-based foods, as well as supplements designed to optimise your immunity.

What negatively affects immunity?

The health of your immune system is inseparable from general health. Factors which negatively affect general health therefore impair the functioning of the immune system. These include poor diet, stress, lack of exercise and bad habits such as smoking and excessive alcohol consumption.

Stress may seem to be the least obvious, and yet it is a major contributor to general negative health, and specifically affects the immune system. In the short term, stress can have a positive effect on immunity, priming the body for attack. Short term stress is therefore beneficial, or at least not harmful. Stress becomes an issue, both for health and immunity, when it is chronic.

Biologically, stress causes a number of changes in the body. Many of these are linked to the secretion of cortisol, a hormone which regulates a variety of processes in the body. Cortisol prepares you for ‘fight or flight’, steering energy away from processes that normally use energy but would not necessarily be useful to helping you survive an attack by a bear, for example. One of the biological functions which these hormones impair is the immune response. In the short term, this is not problematic, but in the long term, if cortisol is secreted in excess, the body can enter a state of chronic immune deficiency. Managing stress is therefore extremely important to ensuring optimal immune function.

Why does exercise supress the immune system?

Regular exercise is important to maintaining health and the immune system. Despite this, excessive amounts of exercise can negatively affect immunity. This is because exercise is a form of stress. During a normal exercise regime, the body has time to recover and return to normal. When exercise is prolonged (over 90 minutes) and extreme, however, the body does not get this opportunity, and it begins to affect the immune system in a similar way to chronic psychological stress. Athletes may also develop deficiencies of certain micronutrients important to the immune system, such as zinc, which is lost through sweat.

The phenomenon is especially observed in endurance and professional athletes, who are more likely to develop upper respiratory tract infections than the general population. Prolonged, intense exercise leads to suppression of the immune system, affecting white blood cell function, lasting up to 24 hours. During this period, athletes are more susceptible to infection. If adequate rest is not taken between sessions, the body is not able to normalise, and a chronic state of immune suppression can develop.

Adequate recovery periods and rest is therefore important, as is focus on good nutrition, high in vitamins and minerals.  It has also been found that maintaining energy levels through consumption of carbohydrates while training can reduce the negative effect on the immune system.

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