There are three main kinds of allergy: allergy to food, inhalant allergy, and contact allergy.

Allergies to pollens and seasonal grasses fall into the inhalant allergy category, because breathing them triggers allergic reactions. Some people have exercise-induced allergies, especially when exercising outdoors, because breathing increases and thus exposure to allergens is enhanced. But you can also inhale allergens indoors. Many people have indoor allergies, the most common ones being to pet dander, mold, and dust mites. Inhalant allergies usually exhibit themselves in breathing difficulties, congestion, watery eyes, sneezing, and other respiratory symptoms.

Contact dermatitis is the official name for allergic reactions to things that come in contact with the skin. Many people don't realize that poison ivy is an allergen. Poison ivy is probably the best known example of contact dermatitis. Poison ivy isn't poisonous; it triggers an allergic reaction in most people. Rashes and skin irritation are the body's way of trying to repel an "enemy" in the form of an allergen. Some people have contact dermatitis reactions due to an allergy to rubber, wool, dyes or cosmetics. Nickel is the most common metal to trigger contact allergies; people with a nickel allergy need to be careful that they don't wear jewelry that contains nickel, in bracelets, necklaces, or in body piercings. Certain cosmetics ingredients trigger allergies, including some ingredients in hair dyes. Fragrance is the most common cause of allergies to cosmetics; if you experience allergies, try unscented products. Contact allergies typically are manifested in itching, rashes and hives.

Not all contact dermatitis is due to allergy. In fact, 80 percent of skin reactions are to actual irritants such as chemicals, acids, and some soaps.

Food allergies typically manifest themselves with itching, rashes and hives, but they can also affect the respiratory system. In severe cases, a food allergy can trigger anaphylactic shock that causes the throat to close and breathing passages to swell. Food allergies usually become obvious within two hours of consuming the food allergen.