Mast cell

The mast cell, also called the mastocyte, is an immune system cell that arises from the myeloid lineage, the branch of the bone marrow that also produces several kinds of white blood cells. It has a quirk: once mature it exists only inside tissues, and unlike other defense cells it never travels around in the blood.

It lives in loose connective tissue, the soft packing that supports and separates structures, and it is present in practically every organ. Inside, it holds many secretory granules, small packets loaded with active substances, and on the outside, its membrane carries receptors for immunoglobulin E, an antibody tied to allergy.

Setting it off takes a very specific condition. When that membrane-bound immunoglobulin binds a foreign body and two of its receptors end up cross-linked, the mast cell degranulates: it dumps its packets all at once into the surrounding space. It is a loaded bag with the safety on: it takes two keys at once to open it. The most notable of those substances is histamine, which draws white blood cells to the site, contracts the smooth muscle of the airways and makes blood vessels more permeable.