Allergies
Definition
An allergy is an exaggerated immune response or reaction to substances that are generally not harmful.
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Causes
Allergies are relatively common. Both genetics and environmental factors play a role.
Allergy is caused by an oversensitive immune system, which leads to a misdirected immune response. The immune system normally protects the body against harmful substances, such as bacteria and viruses. It reacts to substances (allergens) that are generally harmless and in most people do not cause a problem.
But in a person with allergies, the immune response is oversensitive. When it recognizes an allergen, it releases chemicals, such as histamines. This causes itching, swelling, mucus production, muscle spasms, hives, rashes, and other symptoms, which vary from person to person.
What part of the body is contacted by the allergen plays a role in the symptoms you develop. For example, allergens that are breathed in often cause a stuffy nose, itchy nose and throat, mucus production, cough, or wheezing. A food allergen can cause nausea, vomiting, abdominal pain, cramping, diarrhea, or a severe, life-threatening reaction. Allergies to plants often cause a skin rash. Drug allergies usually involve the whole body and can lead to a variety of symptoms.
Some medical conditions, such as eczema and asthma, are linked to allergies.
Common allergens include pollen, mold, pet dander, and dust. Food and drug allergies are common. Allergic reactions can also be caused by insect bites, jewelry, cosmetics, and other substances.
Some people have allergy-like reactions to hot or cold temperatures, sunlight, or other physical stimuli. Sometimes, friction (rubbing or vigorously stroking the skin) will cause symptoms.
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References
Christodoulopoulos P, Tulic MK, Kontolemos M, Hamid Q. Immunopathology of Allergic Airway Inflammation.In: Middleton's Allergy: Principles and Practice. 6th ed. St. Louis, Mo: Mosby; 2003; 501-511.
Goldman L, Ausiello D. Cecil Textbook of Medicine, 22nd ed. Philadelphia, Pa: WB Saunders; 2004:1590-1591.
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