Allergies are caused by an oversensitive immune system, leading to a misdirected immune response. The immune system normally protects the body against harmful substances such as bacteria and viruses. Allergy occurs when the immune system reacts to substances (allergens) that are generally harmless and in most people do not cause an immune response.
An allergy to airborne particles most commonly leads to allergic rhinitis (symptoms in the eyes, nose, and throat). Hay fever is the term used for an allergic reaction to pollen. However, people may develop an allergic reaction to other particles in the indoor or outdoor environment that leads to symptoms virtually identical to hay fever. - Mold -- Mold spores are carried in the air and may be present all year long. Mold is most common indoors in damp locations such as swamp coolers, basements, bathrooms, or washrooms. Fabrics, rugs, stuffed animals, books, or wallpaper can harbor mold spores if they are frequently in contact with water or kept in a damp place. Outdoors, mold lives in the soil, on compost, and on damp vegetation.
- Animals -- People who are allergic to certain animals are, in fact, rarely allergic to the animals' fur or feathers themselves. They are actually allergic to the small scales of skin (dander) that the animal sheds. Some people are allergic to the animal's saliva, particularly cats (whose saliva contains a protein known to cause allergy). Saliva exposure occurs if the animal licks the person. It may also occur from petting the animal after it has groomed itself or from touching an object that the animal has recently licked or chewed.
- Dust -- House dust contains microscopic particles of pollen, mold, fibers from clothing and other fabrics, detergents, and microscopic insects (mites). Dust mites, including small fragments of dead mites, are the primary cause of dust allergy and are found in the highest numbers in bedding, mattresses, and boxsprings.
A few people develop allergies to other irritants in the environment, including smoke, fumes from industries or cleaning products, tobacco, powder (face powder, baby powder, and so on), laundry detergents, and other common substances.
When an allergen enters the body of a person with a sensitized immune system, it triggers antibody production. Histamine and other chemicals are released by body tissues as part of the immune response. This causes itching and swelling in affected tissues, mucus production, and in serious cases, hives and rashes, as well as other symptoms.
Symptoms vary in severity from person to person. Most environmental allergens contact the skin or eyes, or are inhaled. Therefore, most symptoms affect the skin, eyes, or the breathing passages.
Many disorders are associated with, triggered by, or worsened by allergies. These include eczema and asthma.
Allergies are relatively common. Heredity, environmental conditions, number and type of exposures can affect a predisposition to allergies. For reasons that are not fully clear to scientists, allergies are on the rise, particularly in so-called "Westernized" regions such as the United States and Europe.
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