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. A specific response occurs when the immune system recognises an antigen that does not belong in the body and then prepares a specific reaction to it. For example, if influenza viruses enter the tissues, cells of the immune system will recognise that the surface antigens of the viruses are different from the normal antigens of the body's cells and prepare a response. Here the antibodies will lock onto the viral antigens.