Mast cell: A granulocyte found in connective tissue whose normal function is unknown but that is frequently injured during allergic reactions. When a mast cell is injured, it releases strong chemicals, including histamine, into the tissues and blood. These chemicals are very irritating and cause itching, swelling, and fluid leaking from cells. They can also cause muscle spasm, leading to lung and throat tightening (as is found in asthma) and loss of voice.
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