The Role of Histamine
A big player in the body's defense against these parasite pretenders is histamine, a substance released as part of our immune response. Histamine is a vasodilator, meaning that it opens up blood vessels. "Mast cells that contain histamine line all of the blood vessels and it's part of what regulates the leakiness — or lack thereof — of blood vessels," says White. Histamine also promotes secretion of other fluids. In the case of a parasite, vasodilation and these secretions would help get white blood cells to the problem areas. In the case of hay fever, histamine just makes people a mess.