HistamineThe effect histamine has on blood vessels is crucial to its role in the immune response, which is most clearly observed in inflammation—i.e. the local reaction of bodily tissues to injury caused by physical damage, infection, or allergic reaction. Injured tissue mast cells release histamine, causing the surrounding blood vessels to dilate and increase in permeability. This allows fluid and cells of the immune system, such as leucocytes (white blood cells) and blood plasma proteins, to leak from the bloodstream through the vessel walls and migrate to the site of tissue injury or infection, where they begin to fight the infection and nourish and heal the injured tissues.
In an allergic reaction—the immune system’s hypersensitivity reaction to usually harmless foreign substances (called antigens in this context) that enter the body—mast cells release histamine in inordinate amounts. Immune system proteins called antibodies, which are bound to mast cells, bind to the antigens to remove them, but in the process the mast cells are stimulated to release their histamines. This causes the visible symptoms of a localized allergic reaction, including runny nose, watery eyes, constriction of bronchi, and tissue swelling. Histamine also contributes to generalized allergic conditions such as anaphylaxis, a severe, immediate, and often fatal response to exposure to a previously encountered antigen.