An allergy describes any disorder where a patient has a runaway histamine response to foreign matter entering the body through the skin, digestive tract or mucous membranes. Most allergies are merely a nuisance to the patient, but some reactions can be genuinely life threatening.
Some conditions described as allergies are actually other disorders where exposure to a certain material causes other discomfort in the patient. For example, lactose intolerance is not an allergy (it is caused by a lack of a particular enzyme used to digest the natural sugars in milk), but exhibits many of the same symptoms.
Almost anything can be an allergen - the source of an allergic reaction. Common allergens are:
Natural substances - dust (actually the feces of dust mites), pollen, grass, mold, etc.
Animals - cats (actually, their dander or sloughed off skin cells coated in saliva), dogs, horses, etc.
Drugs - penicillin, etc.
Foods - peanuts, "true" nuts such as hazelnuts, etc.
Insects - bee stings, mosquito stings, etc.
Most allergic reactions are mild and merely result in symptoms such as a runny nose, coughing, or a skin rash. However, in some patients, the histamine response to even a minor amount of an allergen can be overwhelming and cause bronchoconstriction from anaphylactic shock, a potentially life threatening condition which can easily cause the patient to suffocate if not treated immediately.
Minor allergies can be treated with antihistamines, which suppress the body's histamine response and lessen the symptoms. In the long term, immunotherapy can be used to essentially train the body to ignore the effect of the allergen. However, this treatment is not successful in all patients and is dangerous in patients with severe allergies. In cases of anaphylactic shock, adrenaline must be administered which blocks the histamine response and reduces swelling of the tissues immediately. Because quick treatment is often necessary in such cases, persons who have severe allergies often carry a product such as an EpiPen(TM), which when thrust into the thigh automatically delivers an appropriate dose of synthetic adrenaline.