During an allergic reaction, there is an increased release of histamines and serotonin from mast cells and basophils. As degranulation occurs, it causes the mast cell or basophil to release a series of chemicals that orchestrate the allergic reaction. Within every mast cell or basophil are 500 to 1500 granules containing more than thirty different allergy-causing chemicals. The best-known chemicals that are released is histamine and serotonin.