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Your Immune System

The Immune System’s Role in Your Health

Immune defence mechanisms evolved to be able to discriminate between what is ‘self’ and ‘non-self’ to prevent autoimmune-related issues. Even single-celled creatures demonstrated this capability. The immune system of modern man gradually developed into a highly complex system that could adapt rapidly to new dangers, new invaders (pathogens) and new situations. Alongside it, our stress response system was designed to detect and respond to acute danger signals that were a direct threat to our survival. Together, these physiological mechanisms gave us the adaptability that allowed our ancestors to explore the world and inhabit a multitude of different environments.

Danger, Immunity and Our ‘Sixth Sense’

We detect danger through our ﬁve senses. These senses provide information about the world around us. When our senses perceive a threat, a cascade of physiological events is activated, commonly referred to as the ‘ﬁght or ﬂight’ response. In the past, many environmental danger signals would have carried an implied danger of injury and infection, so an immune response was needed, too.

It has been discovered that the immune system and nervous system communicate with one another using a common chemical language. This allows the immune system to act as our ‘sixth sense’ and alert the nervous system to the presence of possible invaders or pathogens. Such pathogens cannot be detected via the usual ﬁve senses. Through its communication with the brain, the immune system can regulate behaviour in a way that is consistent with our survival.

Your Body’s Natural Responses

An acute stress response in the face of danger activates the nervous, hormonal and immune systems. Just as in mankind’s early days, acute stressors still activate the sympathetic nervous system and hypothalamic-adrenal axis (the ‘adrenaline rush’). This process is started in order to increase energy to organs needed for direct survival – the brain for decision making, the heart to pump more blood, the immune system for protection and the muscles for physical action. Organs not immediately required for survival are temporarily closed down to reduce their energy demand; for instance, our digestive system works less efficiently when we are in a stressful situation.

Sickness Behaviour and Modern-day Stressors

When activated, the immune system produces a state of inﬂammation. This triggers signalling across the blood-brain barrier to inform the brain there is an infection or impending threat of infection (3). Inﬂammation is the body’s way of defending itself, isolating and eliminating infection, removing damaged tissue and promoting healing. Inﬂammation also releases substances that act directly on the brain to produce ‘sickness behaviour’ – sleepiness, loss of appetite and becoming sedentary.

Sickness behaviour allows energy to be withheld from organs not involved in the survival response and diverted instead to the innate immune system – the body’s ﬁrst line of defence against an infectious challenge. The innate immune system response should last about 12-28 hours. If the immune challenge is not resolved, the adaptive immune system, the body’s second line of defence, takes over, producing a much more speciﬁc and targeted reaction to individual pathogens. This self-limiting response ends after about 28 days. Inﬂammation is the natural process by which the immune system deals with an acute danger signal. Ancient stressors that directly affected our survival included hunger, thirst, cold, fear and violence. Fast forward half a million years and we see a very different picture. New stress signals arise from processed foods, sedentary lifestyles, environmental pollution and levels of psycho-emotional stress that would rarely have been a part of our early, tribal history.

So, how does the immune system, designed to respond to injury or acute infection in the short term, deal with ongoing worries about paying the mortgage, or social isolation, or high blood sugar and insulin levels from an overload of reﬁned carbohydrates? Quite simply, it can’t! What we see instead is a slower brain-hormone-immune activation and a weakened immune response because our body can’t resolve these 21st century stressors using our age-old mechanisms.

The Smouldering Fire of Chronic Inflammation

Because the immune system has no other means of meeting these new kinds of ongoing stress, it responds with inﬂammation. We can think of this chronic, low-grade inﬂammation as a smouldering ﬁre, which can only be put out when the problems are solved. But the body is unable to produce the substances needed to mount a full inﬂammatory response that comes to a natural conclusion. The innate immune system also has a high demand for energy and nutrients. Even short-term, mild mental or social stress can increase the amount of energy needed by the brain by 12 per cent. So where does this energy come from? The only way an inﬂammatory response can be maintained longer than the maximum 28 days is if the immune system actively takes more energy from other body systems. To do so, it uses the same mechanism that evolved to keep our vital organs functioning during acute danger, by ‘closing down’ other non-vital organs. The thyroid gland, the body’s main regulator of energy metabolism, is able to down-regulate all other organs so that their energy can be re-allocated.

The Selfish Immune System

We can now see that the process of inﬂammation, so useful in one environment, becomes detrimental in another. It is the mechanism through which psycho-emotional stress produces sickness behaviour, memory changes, tiredness and depression, as well as chronic inﬂammatory diseases.

The longer low-grade inﬂammation persists, the more selﬁsh the immune system becomes in demanding an ever greater share of the body’s energy, at the expense of all other organs and body systems. It has been suggested that the brain is selﬁsh and will never relinquish its energy demand. But this is only true up to the point where the immune system takes over the priority. Stress inhibits pancreatic insulin production so that blood sugar remains high, ensuring a good supply for the brain. But now the immune system is able to down-regulate glucose transporter receptors at the blood-brain barrier, as well as in muscle and fat tissue to utilise more of the available glucose itself.

Hormones that also have an immune system function, such as leptin, insulin and cortisol, are diverted for the immune system’s use by various mechanisms, inducing resistance of the rest of the body’s organs to these hormones. Eventually, almost all neurotransmitters are also put at the service of the immune system and so are depleted in the rest of the body. This process is considered to mimic mammalian hibernation; it has been called non-permissive brain syndrome and essentially it is long-lasting sickness behavior. It is present in diseases with disturbed brain function, such as Alzheimer’s disease, Parkinson’s disease, ﬁbromyalgia syndrome, chronic fatigue syndrome and depression.

The Bottom Line

Activation of the stress response and immune system leads to the redistribution of the available energy in the body. The energy favours the brain, other vital organs and the immune system initially. Chronic, weak immune activation by long-term stressors, however, eventually causes the energy demand from the immune system to override the ‘selﬁsh brain.’ But all along the immune system is only doing what it is designed to do – protect us. Sickness behaviour and non-permissive brain syndrome, as a response to chronic immune activation, are the last means of protecting the body in a new environment, from new danger signals to which it has not had time to adapt. Understanding this and the ways in which diet, exercise and stress reduction techniques can help to resolve the situation is key to the clinical psychoneuroimmunology approach to the treatment of chronic disease.

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