Allergens, also called antigens, are substances that can trigger an allergic reaction. Having allergies means that the body reacts to allergens as a harmful foreign substance.

Allergens may be:

• Inhaled in the form of pollens, mould, chemicals, material from the droppings of cockroaches and house dust mites, or animal dander and saliva. Animal dander is dead skin cells from an animal, such as a cat or dog. Animal hair or fur is not an allergen.
• Eaten or taken by mouth, including foods, food supplements, home remedies, or medicines.
• Touched, such as cosmetics, plants, soaps or detergents, chemicals, metals, or latex (causing skin or contact allergies).
• Applied to the skin, such as artificial nails, hair extensions, or henna tattoos.
• Injected under the skin, such as medicines or venom from the bites or stings of bees, yellow jackets, hornets, or other insects.

Allergy symptoms may be controlled by avoiding exposure to allergens. Medicines and immunotherapy may help relieve symptoms or reduce the body's reactions to certain allergens.