One of the most widespread illnesses around, allergies are an overreaction of the immune system to substances – known as allergens – that are otherwise harmless. Allergens are substances that are foreign to the body and can cause an allergic reaction. Strictly speaking, allergy is one of four forms of hypersensitivity, which can result in an extreme inflammatory response. Common reactions include eczema, hives, hay fever, asthma attacks, food allergies, and reactions to the venom of stinging insects such as wasps and bees.