The symptoms of an allergic reaction develop in response to histamine. Mast cells release a large amount of histamine into the bloodstream and it also act as initiator of the inflammatory response, which aids the arrival of leucocytes at a site of infection. Histamine stimulates capillary dilation, increased capillary permeability, closure of bronchial tubes, mucus secretion, pain and swelling. Hence, both Assertion and Reason are true and Reason is the correct explanation of Assertion.