- Diseases, Disorders & Conditions
Allergy Causes and Treatment
The word "allergy" is a term coined by von Pirquet in 1906 to describe the altered reaction of an animal to a foreign substance following previous exposure to it, and now used to refer to the exaggerated reaction by the body to various foreign substances or physical agents that are harmless in similar amounts to normal, non-allergic people.
The basis of allergy is the immune response, a specific reaction in which the body produces antibodies against invading foreign substances (usually proteins) known as antigens. This reaction is normally beneficial, as it protects the body against infection, for instance by neutralizing the antigens of invading bacteria, but on occasion the reaction is so exaggerated as to cause harm to the body itself. Such a reaction is known as an allergic, or hypersensitivity, reaction and it is due to the liberation of pharmacologically active substances following the interaction of antigen and antibody. Much has been discovered about allergic reactions within recent years.
It is now realized that allergic reactions can produce immediate or delayed effects, and that there are various mechanisms by which hypersensitivity can be produced. The most widely accepted classification of hypersensitivity reactions is that of Gell and Coombs, who defined four types.
Type I or anaphylactic reactions are the result of the re-exposure of an individual to an antigen to which he is already sensitive. He shows an immediate reaction, which may either be localized or generalized in its effects, depending on the mode of presentation of the antigen. For example, a patient who is allergic to the antibiotic penicillin responds to an injection of it throughout his entire body, and the resulting severe reaction may kill him. On the other hand, an asthmatic patient who is sensitive to grass pollen experiences a localized reaction in his lungs as a spasm of the bronchial tubes.
The other symptoms experienced include itching, running eyes, and swelling of tissues, and all these symptoms are explained by the release of pharmacologically active substances from cells following the antigen-antibody reaction. These substances are histamine, serotonin, slow reacting substance, and bradykinin. The liberation of histamine has two important physiological actions; first, it causes the small blood vessels to leak so that fluid is poured out into the tissues (the effect may be so pronounced that the blood pressure may fall and unconsciousness ensue; this is called anaphylactic shock). Secondly, it causes spasm of smooth muscle, for example, of the lungs. Observation of the symptoms of hay fever and asthma shows that these actions do indeed take place.
Type II, III, and IV reactions are more specialized forms of an allergic or hypersensitive reaction, of which respective examples are the results of an incompatible blood transfusion; serum sickness following immunization with serum prepared in an animal; and the tuberculin test for immunity to tuberculosis. The latter is an example of delayed hypersensitivity, for the reaction is not apparent for many hours and the effects persist for some weeks.
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