Mast cells secrete:

Mast cells are found in mucous membranes and connective tissues and are important for wound healing and defense against pathogens via the inflammatory response. The mast cell granules are composed of heparin and histamine. They secrete heparin, and histamine when they are activated due to toxic substance or pathogen. When released histamine cause blood vessels to dilate, increasing blood flow and cell trafficking to the area of infection. It is also involved in the inflammatory response. serotonin, a protein which acts as a vasoconstrictor to stop bleeding and to increase blood pressure is also released from mast cells. Related Theory As part of an immune response to foreign pathogens, histamine is produced by basophils and by mast cells found in nearby connective tissues. Histamine increases the permeability of the capillaries to white blood cells and some proteins, to allow them to engage pathogens in the infected tissues.