Mast cell

A mast cell is an immune cell that lives in connective tissue rather than circulating in the blood. Mast cells arise from bone marrow precursors that travel through the blood in an immature form and finish maturing once they reach the tissues. There they sit near blood vessels and nerves and close to the surfaces that face the outside world, such as the skin, the airways and the gut lining. Their cytoplasm is crowded with granules holding histamine, heparin, protein-cutting enzymes and other chemicals. They resemble basophils of the blood, but the two are separate cell types.

Mast cells carry high-affinity receptors that hold IgE antibodies on their surface. When an allergen or a parasite antigen cross-links those antibodies, the cell degranulates, emptying its stored granules within minutes and then making fresh mediators such as leukotrienes, prostaglandins and cytokines. These widen and loosen blood vessels, tighten airway muscle and call in other white blood cells. The response helps fight parasites and some bacteria, but it also produces the symptoms of hay fever, hives, allergic asthma and anaphylaxis.

Paul Ehrlich described the cells in the 1870s and named them from the German Mast, fattening, because he thought their granules fed the tissue. Mast cell stabilizers such as cromolyn work by holding back degranulation.