Stress: Fight or Flight & The HPA Axis, by Dr Nina Bailey


The “fight or flight response” is the body’s primitive and automatic response which prepares the body to either “fight” or “run” from a perceived attack or, in other words, it is our biological response to acute stress. During a stressful experience there is a complex set of interactions between the hypothalamus (a part of the brain), the pituitary gland (also part of the brain) and the adrenal glands (at the top of each kidney). Several types of neurotransmitters are involved in this system, collectively known as the hypothalamo-pituitary-adrenocortical- (HPA) axis, which plays a huge role in mood (anxiety, bipolar disorder, insomnia, depression) and many physical disorders including ADHD, chronic fatigue syndrome, fibromyalgia and irritable bowel syndrome. Because these illnesses are associated with neurotransmitter function it is not surprising that antidepressants are routinely prescribed.

The immune response

The immune response is our defence mechanism against bacteria, viruses, or substances that that are not recognised by the body and are capable of causing us harm. The immune system is made up of different types of white cells all having a different function. Some of these cells directly kill bacteria and viruses, whilst other cells produce chemicals that act as signalling molecules to send messages to other white cells telling them what to do. During an immune response the body produces substances called cytokines which signal certain immune cells to travel to the site of infection. Cytokines can be inflammatory or anti-inflammatory, either activating or turning cells ‘off’. Certain inflammatory cytokines, however, have the ability to activate the HPA-axis and therefore have been implicated in both mood and physical disorders. It can also be a vicious circle in the sense that cytokines can also make these cells produce more cytokines, compounding the problem. As long as this system is kept under control, infections can be dealt with and the body returns to normal. On the other hand if the system becomes over stimulated, too many immune cells are activated in a single location and cytokine levels can become imbalanced. Certain proinflammatory cytokines have the ability to activate the HPA axis, and there is widespread evidence suggesting that such cytokines may affect the axis at multiple levels.

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