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What is an allergy?

If someone breaks out in a rash or is made to sneeze by such things as dust, animal fur or pollen, then he or she has an allergy.

To have an allergy means to be affected by something which is harmless to most people. The substance causing the allergic reaction is called an allergen. People can be allergic to all sorts of things, including certain foods.

The reason why some people have allergies while others do not is little understood. Many doctors and scientists think that heredity has a part in deciding who will be allergic, because there are families in which grandparents, parents and children are all allergic to the same substance. However, in many other cases of allergy no other member of the family has been allergic. While doctors do not know why certain people have allergies, they do know something about how the body behaves when in contact with the substance which starts the attack.

For example, suppose a child is allergic to chocolate. The first time he eats chocolate, it is digested in the stomach and intestines. The digested chocolate causes special cells in the tissues of the body to make a chemical called an antibody. The next time chocolate is eaten, its digestive products combine with the antibodies in the skin and lungs. This causes damage to the small blood vessels in the skin, allowing fluid to escape. As a result, little spots of fluid collect on the skin, appearing as hives (spots or blotches). The effect on the lungs may be a pouring out of thick mucus and the closing down of the bronchial tubes. This is asthma : during the attack the child has great difficulty in breathing.

Antibodies in general are among the body's main defenses against infection. It is only certain antibodies which cause harm in this way. When this special kind of antibody combines with the allergen (in this case, chocolate), a substance called histamine is released. The histamine then causes the allergic reaction. Certain drugs, called antihistamine drugs, can prevent the allergic reaction though they cannot cure the allergy.

Hay fever is a common allergy.

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