Overview of Allergic Reactions - Overview of Allergic Reactions - Merck Manual Consumer Version

Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance.Usually, allergies cause sneezing, watery and itchy eyes, a runny nose, itchy skin, and rash.Some allergic reactions, called anaphylactic reactions, are life threatening.Symptoms suggest the diagnosis, and skin testing can help identify the substance that triggers the allergy but does not predict the severity of a future reaction. Avoiding the trigger is best, but if it is impossible, allergy shots, when given long before the exposure occurs, can sometimes desensitize the person.Severe reactions require emergency treatment in an emergency care facility.Normally, the immune system—which includes antibodies, white blood cells, mast cells, complement proteins, and other substances—defends the body against foreign substances (called antigens). However, in susceptible people, the immune system can overreact when exposed to certain substances (allergens) in the en...