They are able to release potent inflammatory mediators, such as histamine, proteases, chemotactic factors, cytokines and metabolites of arachidonic acid that act on the vasculature, smooth muscle, connective tissue, mucous glands and inflammatory cells. Mast cells settle in connective tissues and usually do not circulate in the blood stream.
Basophils are the smallest circulating granulocytes with relatively the least known function.
They arise in the bone marrow, and following maturation and differentiation, are released into the blood circulation. If they are adequately stimulated they may settle in the tissues. Both mast cells and basophils contain special cytoplasmic granules which store mediators of inflammation. In the IgE-independent way, the anafylatoxins C5a, C3a and C4a are formed during activation of complement. Then, the degranulation is triggered through C5a-receptors on the surface of mast cells and basophils.