An allergic reaction occurs when the immune system attacks a usually harmless substance called an allergen that gains access to the body. To most simply describe this complex immune process, we will make an analogy to a war within the body. The immune system calls upon a protective substance called immunoglobulin E antibodies ("E" for "erythema") to fight these invading allergic substances or allergens. Even though everyone has some IgE, an allergic person has an unusually large army of these Ige defenders -- in fact, too many for his 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 in battle. These innocent standers are special cells called mast cells. These cells are frequently injured during the warring of the IgE antibodies and the allergic substances. When a mast cell is injured, it releases a variety of strong chemicals into the tissues and blood, one of which is known as histamine. These chemicals frequently cause allergic reactions. These chemicals are very irritating and cause itching, swelling, and fluid leaking from cells. Through various mechanisms, these allergic chemicals can reasons muscle spasm and can lead to lung and throat tightening as is found in asthma and loss of voice.
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