Allergies are a reaction by the body's immune system to normally harmless substances that it sees as harmful. The allergy-causing substances are called allergens.

Your immune system is your body's natural defense against infection and other foreign material. Before you can have a reaction to a particular substance, your immune system must first be sensitized to it. This means your body has to have been exposed to the substance at least once before. Once sensitized, your body will react every time you have contact with that substance. Many substances can cause an allergic reaction. The most common are pollen, mold, animal dander, dust and dust mites, latex, medicines, insect stings, and foods.

The allergens may cause different kinds of allergic reactions. The most common allergic conditions are hay fever, asthma, and skin allergies.

• Airborne allergens such as mold and the pollen of trees, grasses, and weeds cause hay fever.
• Pollens, molds, and house dust can trigger asthma attacks.
• Allergic reactions of the skin can have many possible causes. Examples of irritants that can cause allergic reactions when they touch your skin are hair or skin care products, nickel in jewelry and belt buckles, dyes in leather or fabric, and poison ivy or poison oak. Eczema is a skin condition that causes itching, dryness, fine scales or flaking, and sometimes mild redness. The cause of eczema is usually not known.
• It is also possible to have an allergic reaction to sunlight or temperature extremes.
• Common foods that may cause allergy symptoms are shellfish, eggs, milk, nuts, and peanuts. Food allergies often occur in children, who may outgrow them.

It is not known why some people develop allergies to certain substances. Allergies run in families, but not every family member may be allergic to the same thing.

Sometimes an allergic reaction may be severe. This is called anaphylaxis. It is a life-threatening emergency that can affect breathing and circulation within several minutes. Insect stings, certain foods, and drugs such as penicillin are some of the more common causes of severe allergic reactions.