Allergies - Causes



When allergens enter the body, they set off a series of reactions. The first of these reactions occurs when the allergens come into contact with lymphocytes (pronounced LIM-fuh-sites), a type of white blood cells. Lymphocytes produce a special kind of marker molecule known as immunoglobulin (pronounced IM-yuh-no-GLOB-yuh-lin) type E (IgE). IgE molecules search out and attach themselves to a second kind of cell, called mast cells.

Mast cells manufacture a chemical known as histamine (pronounced HISS-tuh-meen). When IgE molecules attach themselves to a mast cell, the cell begins to release large amounts of histamine into blood vessels, nerves, and other tissue around it. Histamine causes changes in the cells with which it comes into contact. For example, it causes blood vessels to become leaky. Fluid escapes from the blood vessel, causing swelling and redness in surrounding tissue.

Histamine also causes nerve cells to become more sensitive, producing pain and irritation. All of these changes result in the familiar symptoms of an allergic reaction: redness, swelling, pain, and itching.

Common Allergens

The most common allergens in the air are:

  • Animal fur and dander (dry skin that is shed)
  • Cigarette smoke
  • Dust
  • House mites
  • Mold spores (seeds)
  • Plant pollen
  • Solvents (chemicals used in cleaning)

Common food allergens include:

  • Eggs
  • Fish and shellfish
  • Food additives
  • Milk
  • Nuts, especially peanuts, walnuts, and Brazil nuts
  • Wheat

Specific kinds of allergies are not inherited. But the tendency to develop some kinds of allergies can be. This pattern is thought to result from the body's tendency to produce IgE. The more IgE the body produces, the more likely a person is to have allergies. IgE is a hereditary trait. Some people's bodies naturally produce more or less of the chemical, a trait they can pass on to their children.

If neither parent has allergies, the chance of a child developing an allergy is no more than 20 percent. If one parent has allergies, the chance rises to as high as 50 percent. If both parents have allergies, the chance can be as high as 75 percent.

The following types of drugs often cause allergic reactions:

  • Flu vaccines
  • Gamma globulin (pronounced GA-muh GLAH-byu-lun), which are used to treat infectious diseases such as measles (see measles entry) and hepatitis (see hepatitis entry)
  • Penicillin and other antibiotics Common causes of contact dermatitis include the following:
  • Latex products (such as rubber gloves)
  • Nickel or nickel alloys
  • Poison ivy, poison oak, and poison sumac

Insects or other animals whose bites or stings can cause allergies include the following:

  • Bees, wasps, hornets, and yellow jackets
  • Fleas
  • Mosquitoes

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