The immune system works, particularly like an armed force. Certain cells frame an order post to coordinate the exercises of the fighter cells. Two types of troops are mobilized: killer T cells and antibodies. These go into action depending upon the type of invader that is mounting an attack on your body. Killer T cells are employed to fight against viruses. Viruses penetrate and hide inside your body cells so the cells in which they are residing need to be destroyed. If the invaders are bacteria, the antibodies go into action. Bacteria do not invade cells but instead stay outside. The antibodies surround, immobilize and destroy the bacteria.