Triggering of mast cells.

Mast cells secrete histamine, glycosaminoglycans, arachidonic acid derivatives, enzymes, and possibly whole granules. Physiologic stimuli include the bridging of membrane-bound IgE molecules by antigen, the anaphylatoxins C5a and C3a, and the neurotransmitter acetylcholine. Evidence is presented that a 'spontaneous' histamine release may be of biological significance. While the actual trigger of this process is unknown, it appears to be regulated by the concentration of histamine in the environment. It is suggested that the spontaneous release is responsible for maintaining a certain histamine concentration in the body fluids. After mast cell stimulation by IgE cross-linking or drugs changes in lipid metabolism, an influx of Ca2+ ions into the cell and fusions of the perigranular and the cytoplasmic membranes are observed. The physiologic role of the mast cell and its mediators is still a mystery.