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One of the main chemicals to be released is called histamine -hence the use of drugs that counteract its effects, antihistamines, in the treatment of allergies. The packets of chemicals inside the resting mast cell look like small granules under the microscope, so the process of releasing the chemicals is called degranulation.
Histamine and other chemicals released by mast cells are called mediators because they bring about or ‘mediate’ changes in the body. A powerful cocktail of mediators, containing ten or more separate substances, spills out of a degranulating mast cell. Each mediator has its own particular effect on the body – some make the blood vessels open out, others make them more leaky so that blood escapes through the vessel wall. Several mediators make smooth muscles contract – these are not the muscles by which we move around, but those that operate our lungs, stomach, intestine and bladder. When they contract sharply, air may be expelled from the tubes leading to our lungs, or semi-digested food from our bowels.
This is bad news for parasites, which may be directly affected by the mediators themselves, and then assaulted by the body’s reaction to the mediators. In the case of parasites in the gut, for example, the direct effect of the mediators may make the parasites loosen their grip, and the diarrhoea that follows flushes them out of the body. For parasites in the blood, the expansion and leakiness of blood vessels produces the redness and swelling that we describe as inflammation. One feature of inflammation is that all-purpose defensive cells called phagocytes (which simply means ‘eating cells’) are attracted to die site of the invasion.
One group of phagocytes, the macrophages (‘big eaters’) have the role of perpetuating the inflammation reaction. They produce an enzyme called phospholipase or PLA. What PLA then does is to cut up certain fat molecules – the phospholipids – found in the membranes of all our body cells. The fragments released from the phospholipids by PLA are then worked on by other enzymes, which turn them into potent chemical mediators, known as prostaglandins.
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