Cells known as mast cells and basophils mop up some of the antibodies and attach them to their surface, making them highly sensitive to future exposure to the allergen. If the allergen is encountered again, the mast cells and basophils release large quantities of powerful signalling molecules such as histamine, leucotrienes, heparin, and platelet activating factors. These substances cause dilation of local blood vessels, damage to local tissues, loss of fluid from capillaries into the tissues, and contraction of local smooth muscle. Histamine released into the circulation causes peripheral vasodilation and loss of plasma into the tissues. People experiencing this reaction may die of circulatory shock within minutes unless noradrenaline is given to counteract the effects of the histamine.
Antihistamines provide a suitable treatment.
Histamine released in response to exposure to the allergen causes fluid leakage into the tissues and produces a runny nose, runny eyes, and sneezing. The substances released from the mast cells cause spasm of the smooth muscle surrounding the bronchioles, and breathing through the constricted airways becomes very difficult. Antigens are generally proteins and other large organic molecules. Because of their size and complexity, each macromolecule has a highly specific shape. Cells of the immune system are able to recognise these differently shaped antigenic molecules, and if they encounter antigens that are foreign to the body's own repertoire they will mount an immune response.