A mast cell is part of a group of cells called leucocytes. Leucocytes are white blood cells and are found in the blood plasma with erythrocytes, red blood cells. Mast cells have immunological functions, or they are part of the immune system. They form part of an early warning system. When stimulated, they release chemicals that signal either injury or infection and cause an inflammation in the area.
The chemicals that are produced by a mast cell are called mediators. Two common mediators are histamine and heparin. Histamine, the most important chemical mediator, causes capillary walls to become more permeable, or let substances through. Heparin prevents blood from clotting to allow blood to flow to the area of infection or injury. Mast cells play an important role in allergic reactions because of their ability to produce and release histamine.
During an immune response, a mast cell is stimulated by a specific type of antibody, called IgE or immunoglobulin E. Antibodies are grouped into classes based on a chemical chain, or tail, attached to them. There are five classes of antibodies based on the specific amino acid sequence of the chains, A, D, E, G and M. All antibodies are called immunoglobulins, so they are referred to as IgA, IgD, etc.
IgE antibodies attach to the outside of mast cells. All antibodies are specific to particular antigens. The antigen-binding area of the antibodies is left free when they bind to a mast cell. When the mast cell with the antibody attached encounters the specific antigen, the mast cell is stimulated to release histamine.
Histamine is not only released due to encountering a toxic substance, it is also released when mast cells detect injury. It causes nearby blood vessels to dilate allowing more blood to reach the site of the injury or infection. The blood plasma is rich in antibodies and other cells of the immune system. In this way, the mast cells act as an alarm system for the immune cells, attracting them to the required area of infection or injury. The fluid that leaks into the area is what causes swelling during an infection.
Sometimes the body overreacts to foreign substances, which are in fact harmless. Most allergic reactions are due to uncontrolled histamine release when the immune system malfunctions. Symptoms of an allergic reaction are well known, but the underlying cause is less clear. Many of the symptoms of allergies can be attributed to histamine, so what is clear is that mast cells are involved. Anti-histamines block histamine receptors on tissues reducing the effect of histamine on those cells and the subsequent allergic symptoms.