Mast cells carry a huge number of internal vesicles full of inflammatory response mediating chemicals, including the potent vasodilatory and chemotactic chemical histamine. These cells originate from the bone marrow but migrate away to other tissues before reaching maturity. This process is often completed in the skin, gut, and lungs.

The granule load of mast cells is usually so high that when histologically stained the granules almost completely obscure the view of the cells' nuclei; this is a very useful diagnostic and identifying feature when analyzing patient blood smears.