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The hypersensitive response of Allergic reaction: The hypersensitive response of the immune system of an allergic individual to a substance. When an allergen enters the body, it causes the body's immune system to develop an allergic reaction in a person with an allergy to it. This reaction can occur when the immune system attacks a normally harmless substance (the allergen). The immune system calls upon a protective antibody called immunoglobulin E or IgE to fight these invading substances. Even though everyone has some IgE, an allergic person has an unusually large army of these IgE defenders -in fact, too many for their own good. This army of IgE antibodies attacks and engages the invading army of allergic substances of allergens. As is often the case in war, innocent bystanders are affected by this battle. These innocent bystanders are special cells called mast cells. When a mast cell is injured or irritated, it releases a variety of strong chemicals, including histamine, into the tissues and blood that promote allergic reactions. These chemicals are very irritating and cause itching, swelling, and fluid leaking from nearby cells. These allergic chemicals can cause muscle spasm and can lead to lung airway and throat tightening as is found in asthma and loss of voice. They are also what leads to the familiar hay fever or allergic rhinitis and common pink eye.