• Types of allergy
• Words to Know
• Role of immune system
• Anaphylaxis
• Diagnosis and treatment

An allergy is an extreme or overly sensitive response of the immune system to harmless substances in the environment. The immune system launches a complex series of actions against an irritating substance, referred to as an allergen. This immune response may be accompanied by a number of stressful symptoms, ranging from mild to severe to life-threatening. In rare cases, an allergic reaction leads to anaphylactic shock—a condition characterized by a sudden drop in blood pressure, difficulty in breathing, skin irritation, collapse, and possible death.

The immune system may produce several chemical agents that cause allergic reactions. One example...