Allergy, a condition of hypersensitivity in certain persons or animals to substances harmless to most individuals. Some people have characterized allergy as immunity “gone wrong.” In the immune reaction, contact with a disease-producing microorganism or a toxin prompts an individual to build up antibodies (proteins related to globulin serum) against the offending organism or toxin so that he or she will be protected against further exposure. All normal people are able to produce such protective antibodies, but in some the capacity to differentiate potentially harmful substances from harmless ones is absent. These persons produce antibodies against one or many inoffensive substances and thus are said to be allergic. When an antibody reacts with an antigen (a substance that stimulates the formation of antibodies) an allergic reaction results. The symptoms of that reaction will depend on where it takes place. If it occurs in the nose, it may cause sneezing and running of the nose, giving rise to hay fever. In the air passages it may cause contraction, leading to wheezing, coughing, and difficulty in breathing, as in asthma. In the skin, it may produce itching spots, hives, or welts (urticaria). If the reaction takes place in the circulating blood, a severe reaction known as serum sickness may ensue. The allergen, the substance producing the reaction, is usually a protein or protein-carbohydrate complex. It may be inhaled, as dust or pollen; it may be eaten, as eggs or shellfish; it may be injected, as penicillin; or it may act by mere contact, as wool, adhesive tape, or metal.
The variety of substances to which a person may be allergic is almost infinite; diagnosis involves discovering the particular substance or substances to which the patient is hypersensitive (reacting excessively). A careful history of the development of the allergic reaction may give a clue, particularly when it is seasonal, when it is associated with an exposure to a specific substance, or when it occurs only in a particular place. Often it is possible to remain unaffected merely by avoiding the particular allergen to which the person reacts allergically, but common allergens such as dust or pollen cannot easily be avoided. An allergic individual may develop new hypersensitivities, or old hypersensitivities may die out. Allergies usually first appear in childhood, adolescence, or early adulthood, but may develop for the first time later in life.