Medical Specialties: Allergy/immunology, Dermatology
An allergy is an abnormal or hypersensitive immune reaction to an allergen, a substance that is normally harmless, but that triggers an allergic response, including the release of the chemical histamine, which can cause reactions like sneezing or a rash.
If you have an allergy, or are allergic to something, your immune system “believes” that a normally harmless substance — like dust or pollen — is an invader. This allergen can trigger matching antibodies in immune cells, leading to a reaction mediated by release of the chemical histamine and other chemical signals. It is these chemical signals that cause the physical reaction, which can range in severity from sneezing and rashes to whole-body allergic reactions that cause a person's throat to swell, dangerously constricting their breathing.