Why do we sometimes feel the need to scratch ourselves?

Scratching - one of the commonest causes of an itchy skin is an allergy. The word was coined in 1906 by Professor Clemens von Puquet of Vienna to describe a special state of exceptional sensitivity of the body to certain substances brought into contact with it. Such substances are called allergens, and include furs, feather, foods, dust, pollens and drugs. Most people have experienced allergy in some degree at some time, and about 10 percent of the population show more or less permanent symptoms of allergic illnesses. An allergic reaction is always the same, no matter what has caused it. The reason is thought to be that a process similar to the antibody reaction is set off, not in the bloodstream but on the surface of the body cells. This allergen antibody damages the cell walls and sets free a substance called histamine which produces two responses. It allows fluid to escape from the blood vessels into the surrounding tissues and it brings about an involuntary contraction or spasm of certain muscles.
When contact with an allergen is external we develop an itch which may be caused by light, heat, cold, hair or fur. It can also be caused by eating foods such as shellfish, mushrooms and strawberries or by an allergy to some drugs and medicines. If the allergen is inhaled there may be, as in hay fever, an excessive secretion of mucous, or, as in asthma, a severe spasm of the lung's air passages. A true allergen is always a protein, a complex substance which forms an essential part of animal and plant tissues, but the abnormal reaction is produced only by a particular substance or group of substances.

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