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What Causes Allergies?

You have an allergy when your body overreacts to things that don't cause problems for most people. These things are called allergens. Your body's overreaction to the allergens is what causes symptoms (see the box below for a list of symptoms). For example, sometimes the term "hay fever" is used to describe your body's allergic reaction to seasonal allergens in the air, such as grass or pollen.

Your doctor may want to do an allergy skin test to help determine exactly what is causing your allergy. An allergy skin test puts tiny amounts of allergens onto your skin to see which ones you react to. Once you know which allergens you are allergic to, you and your doctor can decide the best treatment. Your doctor may also decide to do a blood test, such as the radioallergosorbent test (called RAST).

Common Allergy Symptoms

  • Runny nose
  • Watery eyes
  • Itchy nose, eyes, and roof of mouth
  • Sneezing
  • Stuffy nose
  • Pressure in the nose and cheeks
  • Ear fullness and popping
  • Dark circles under the eyes
  • Hives

What are the Most Common Allergens?

  • Pollen from trees, grass, and weeds - Allergies that occur in the spring (late April and May) are often due to tree pollen. Allergies that occur in the summer (late May to mid-July) are often due to grass and weed pollen. Allergies that occur in the fall (late August to the first frost) are often due to ragweed.
  • Mold - Mold is common where water tends to collect, such as shower curtains, window moldings, and damp basements. It can also be found in rotting logs, hay, mulches, commercial peat moss, compost piles, and leaf litter. This allergy is usually worse during humid and rainy weather.
  • Animal dander - Proteins found in the skin, saliva, and urine of furry pets such as cats and dogs are allergens. You can be exposed to dander when handling an animal or from house dust that contains dander.
  • Dust - Many allergens, including dust mites, are in dust. Dust mites are tiny living creatures found in bedding, mattresses, carpeting and upholstered furniture. They live on dead skin cells and other things found in house dust.

Things That Can Make Your Allergy Symptoms Worse

  • Aerosol sprays
  • Air pollution
  • Cold temperatures
  • Humidity
  • Irritating fumes
  • Tobacco smoke
  • Wind
  • Wood smoke