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. Keeping a record of your allergy symptoms over a period of time can help you and your doctor identify which allergens cause you to overreact.

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 radio allergosorbent test (called RAST).

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.

Common Allergy Symptoms:

Things that can make your allergy symptoms worse

How can I avoid allergens? Pollens. Shower or bathe before bedtime to wash off pollen and other allergens in your hair and on your skin. Avoid going outside, especially on dry, windy days. Keep windows and doors shut, and use an air conditioner at home and in your car.

Mold. You can reduce the amount of mold in your home by removing houseplants and by frequently cleaning shower curtains, bathroom windows, damp walls, areas with dry rot and indoor trash cans. Use a mix of water and chlorine bleach to kill mold. Open doors and windows and use fans to increase air movement and help prevent mold. Don't carpet bathrooms or other damp rooms and use mold-proof paint instead of wallpaper. Reducing the humidity in your home to 50% or less can also help. You can control your home air quality by using a dehumidifier, keeping the temperature set at 70 degrees, and cleaning or replacing small-particle filters in your central air system.

Pet dander. If your allergies are severe, you may need to give your pets away or at least keep them outside. Cat or dog dander often collects in house dust and takes 4 weeks or more to die down. However, there are ways to reduce the amounts of pet dander in your home. Using allergen-resistant bedding, bathing your pet frequently, and using an air filter can help reduce pet dander. Ask your veterinarian for other ways to reduce pet dander in your home.

Dust and dust mites. To reduce dust mites in your home, remove drapes, feather pillows, upholstered furniture, non-washable comforters and soft toys. Replace carpets with linoleum or wood. Polished floors are best. Mop the floor often with a damp mop and wipe surfaces with a damp cloth. Vacuum regularly with a machine that has a high-efficiency particulate air (HEPA) filter. Vacuum soft furniture and curtains as well as floors. Install an air cleaner with a high-efficiency particulate or electrostatic filter. Wash carpets and upholstery with special cleaners, such as benzyl benzoate or tannic acid spray. Wash all bedding in hot water (hotter than 130°F) every 7 to 10 days. Don't use mattress pads. Cover mattress and pillows with plastic covers. Lower the humidity in your home using a dehumidifier.

What medicines can I take to help relieve my symptoms? Antihistamines help reduce the sneezing, runny nose and itchiness of allergies. They're more useful if you use them before you're exposed to allergens. Some antihistamines can cause drowsiness and dry mouth. Others are less likely to cause these side effects, but some of these require a prescription. Ask your doctor which kind is best for you.

Decongestants, such as such as pseudoephedrine and phenylephrine help temporarily relieve the stuffy nose of allergies. Decongestants are found in many medicines and come as pills, nose sprays and nose drops. They are best used only for a short time. Nose sprays and drops shouldn't be used for more than 3 days because you can become dependent on them. This causes you to feel even more stopped-up when you try to quit using them. You can buy decongestants without a doctor's prescription. However, decongestants can raise your blood pressure, so it's a good idea to talk to your family doctor before using them, especially if you have high blood pressure.

Cromolyn sodium is a nasal spray that helps prevent the body's reaction to allergens. Cromolyn sodium is more helpful if you use it before you're exposed to allergens. This medicine may take 2 to 4 weeks to start working. It is available without a prescription.

Nasal steroid sprays reduce the reaction of the nasal tissues to inhaled allergens. This helps relieve the swelling in your nose so that you feel less stopped-up. Nasal steroid sprays are available with a prescription from your doctor. You won't notice their benefits for up to 2 weeks after starting them. Your doctor may prescribe steroid pills for a short time or give you a steroid shot if your symptoms are severe or if other medicines aren't working for you.

Eye drops. If your other medicines are not helping enough with your itchy, watery eyes, your doctor may prescribe eye drops for you.

What are allergy shots? Allergy shots (also called immunotherapy) contain small amounts of allergens. They're given on a regular schedule so that your body gets used to the allergens and no longer overreacts to them. Allergy shots are only used when the allergens you're sensitive to can be identified and when you can't avoid them. It takes a few months to years to finish treatment, and you may need to have treatments throughout your life.