immunity

B cells, a type of lymphocyte (white blood cell), have an important role in the body's immune system. When a B cell encounters an invading bacterium it starts to divide, forming two different types of cell. One type is a clone of itself that begins to produce antibodies to fight the infection; the other is a memory cell that will persist in the bloodstream, ready to produce antibodies should re-infection occur.

T cells, a type of lymphocyte (white blood cell), have an important role in the body's immune system. When a T cell encounters an invading virus it begins to divide, forming four different types of T cell, each with a different function. Killer T cells destroy cells that have become infected with the virus by lysis. Helper T cells activate more killer T cells and also stimulate B cells to begin antibody production (unlike B cells, T cells do not produce antibodies to destroy invading pathogens). Suppressor T cells protect healthy cells from viral attack and memory T cells persist in the bloodstream to guard against re-infection.

How the body's defence mechanisms keep out invading pathogens or destroy those that succeed in entering the body.

Protection that organisms have against foreign micro-organisms, such as bacteria and viruses, and against cancerous cells (see cancer). The cells that provide this protection are called white blood cells, or leucocytes, and make up the immune system. They include neutrophils and macrophages, which can engulf invading organisms and other unwanted material, and natural killer cells that destroy cells infected by viruses and cancerous cells. Some of the most important immune cells are the B cells and T cells. Immune cells coordinate their activities by means of chemical messengers or lymphokines, including the antiviral messenger interferon. The lymph nodes play a major role in organizing the immune response.

Immunity has been studied mostly in higher vertebrates, especially rodents, domestic animals, and primates.

Immunity is also provided by a range of physical barriers such as the skin, tear fluid, acid in the stomach, and mucus in the airways. AIDS is one of many viral diseases in which the immune system is affected.