A second, specific defense system called immunity is much more selective in its targets. It operates against particular enemies that are identified and remembered by the immune system cells involved. If the enemies return the response is immediate.
The assault troops of specific immunity are white blood cells known as lymphocytes. There are two brigades of these troops, the so-called B cells, and T cells, and their battle tactics are quite different. The spearhead of the T cells brigade, the killer T cells, are close-quarter fighters: they lock on to the enemy, sometimes a cell invaded by a virus, and secrete chemicals to destroy it. They are primed to recognize the antigen or chemical characteristics of the invader. Having attached to the enemy, they emit other substances that summon other T cells and macrophages to the attack.
B cells standoff and fight the invader with antibodies, proteins that exactly complement the chemical structure of the invading antigen. The antibodies combine with the antigen and initiate its destruction. Each B cell can secrete about 2000 antibodies per second.
A certain type of both B cells and T cells remembers any antigen after an initial encounter. If the same organism invades a second time these cells initiate a much swifter reaction than on the first occasion. This process can confer lifetime natural immunity to some diseases, such as measles.
