There can be several different causes for urticaria (also called nettle-rash or hives) of which allergic reactions are just one. Where allergy is at the root of urticaria, it is probably the mast cells found in the lower layers of the skin that cause the problem. When they degranulate, the mediators released have a powerful effect on the tiny blood vessels, or capillaries, that lie all around them in the skin. These capillaries become more leaky, allowing serum (the watery part of the blood) to seep out into the skin itself. This produces the characteristic swellings and itchiness of urticaria.

Where there is a great deal of seepage from the blood vessels, the tissues below the skin may also become filled with watery fluid. This produces a puffi-ness that doctors describe as localized angioedema (or oedema). About 50 per cent of people who are afflicted with urticaria also get angioedema, and some people have angioedema without any urticaria. However, there are a large number of other disorders that can produce angioedema, some of them very serious – they should all be eliminated by a full medical examination, before the possible role of allergy is investigated.

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