Allergy is a disorder of the immune system that is often called atopy. Allergic reactions occur to environmental substances known as allergens; these reactions are acquired, predictable and rapid. Strictly, allergy is one of four forms of hypersensitivity and is called type I (or immediate) hypersensitivity. It is characterized by excessive activation of certain white blood cells called mast cells and basophils by a type of antibody, known as IgE, resulting in an extreme inflammatory response. Common allergic reactions include eczema, hives, hay fever, asthma, food allergies, and reactions to the venom of stinging insects such as wasps and bees.

Mild allergies like hay fever are highly prevalent in the human population and cause symptoms such as allergic conjunctivitis, itchiness and runny nose. Similarly, conditions such as asthma are common, in which allergy plays a major role. In some people, severe allergies to environmental or dietary allergens, or to medication, occur that may result in life-threatening anaphylactic reactions and potentially death.

Allergies occur when a hypersensitive immune system reacts to a common or unusual substance. In some people it is hereditary, but one can also develop an allergy at any time in his or her life. Any substance, whatever its origin, is a possible allergen. Foods, chemicals in foods, medicines, and environmental factors, including pollen, parasites, dust, animal hair, and chemical toxins, for example, can all cause allergic reactions almost anywhere in the body.