An allergy is a condition in which your body's immune system reacts with unusual sensitivity to a certain substance. These offending substances are called allergens, or "triggers". Allergens stimulate your body to produce antibodies that fight the invaders. The antibodies, in turn, cause special cells called mast cells to release a chemical called histamine. An excess of histamine is what results in allergy symptoms. You may be familiar with the term antihistamine, a drug that works to block the excess histamine.
There are many symptoms of an allergic reaction, which can vary greatly depending upon the person and the particular allergen. Common symptoms include respiratory problems such as sneezing, runny nose, or wheezing, as well as itchy, watery, or puffy eyes. These types of symptoms are especially common in hay fever. Some allergies may cause skin symptoms, such as itching, redness, rash, or lumps on the skin. Some allergies can even cause intestinal difficulties like abdominal cramps.
Allergens affect different people in different ways. While some people may be able to cuddle a kitten or walk through a field of fresh-cut grass without difficulty, others may not. Each person has an allergic threshold that allows him or her to tolerate small units of an allergen without manifesting symptoms. However, they can quickly become overloaded by greater exposure, or exposure to a combination of allergens. A person may "grow out of" an allergy or develop a new allergy later in life. It is also believed that emotional factors may trigger allergies.
Some people may develop potentially dangerous, and even fatal, allergies. Insect stings, some food allergens, and certain drugs such as penicillin can cause a condition called anaphylactic shock. If not treated immediately, these people run the risk of unconsciousness, coma, and death. Fortunately, this type of severe allergic reaction is rare.