An allergen is a substance that can cause an allergic reaction in some people. When the immune system recognizes allergens as foreign or dangerous, it reacts by making a type of antibody called immunoglobulin E (IgE) to defend against the allergen. This reaction leads to allergy symptoms. Common allergens include:

• Animal proteins and animal dander
• Dust
• Drugs (such as antibiotics or medicines you put on your skin)
• Foods (such as egg, peanut, milk, nuts, soy, fish, animal meat, and wheat)
• Fungal spores
• Insect and mite feces
• Insect bites and stings (their venom)
• Natural latex rubber
• Pollen

Allergens are usually harmless substances capable of triggering a response that starts in the immune system and results in an allergic reaction. When a person comes in contact with an allergen, the immune system responds by releasing chemicals that typically cause symptoms in the nose, throat, eyes, ears, skin, or roof of the mouth. The severity of the reaction depends on how the person comes in contact with the allergen, and symptoms can range from mild to severe.

It is important to note that an allergen enters the body and is wrongly identified by the immune system as a dangerous substance. In response, the immune system makes antibodies to attack the allergen. These are specific antibodies of the IgE (immunoglobulin E) class. When an allergen is found, IgE antibodies trigger a cascade of immune system reactions, including the release of chemicals known as histamines, which cause allergy symptoms.