The symptoms of some allergic diseases have been described since antiquity, and asthma and eczema were named in the mid-first century BC. Foods, inhalants such as feathers or cat fur, and insect stings were recorded as causing reactions in ancient and more recent times.
It was in the early twentieth century that connections between the immune system and the altered reactions of allergy were made. An immune system operating in this way was termed ‘hypersensitive’. Experiments in the 1920s showed that hypersensitive reactions to pollen or food could be transferred from one person to another, by injecting a small amount of blood serum from the allergic person into another. The previously unaffected person would show reactions to the allergen, demonstrating the involvement of the immune system.
A further major advance took place in the 1960s when IgE, the antibody particularly associated with allergic reactions, was identified. One of the main lines of attack used by the body’s immune system against any substance entering the body is to produce antibodies that bind to the invading substance and activate the rest of the body’s defences by chemical reactions. IgE is one such antibody. Its proper function is to defend the body against parasites, such as worms, which could infest the body. The first time such an invader – called an ‘antigen’ – enters the body, the immune system produces specific IgE antibodies, which then attach themselves to cells in tissues and the bloodstream ready to react should the invader return. There is no reaction as such on the first encounter.
IgE antibodies are primed to bind specifically to the antigen they have already recognised. They are lodged in cells known as ‘mast cells’ in solid tissues, and in ‘basophils’ in the bloodstream. Mast cells are especially common in the breathing passages, lungs, gut and digestive tract, and in the skin. There, the antibodies lie in wait for the returning invader. If it returns in the bloodstream, they recognise it and lock on to the molecule of the antigen, thus triggering chemical reactions and the release of chemicals in the body. ‘Histamine’ is released immediately, while other chemicals – ‘leukotrienes’ and ‘prostaglandins’ – are released more slowly. These chemicals are very powerful and stimulate defence mechanisms to expel invading parasites.
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