The immune response is the body's way of defending itself against foreign substances that invade it to cause infection or disease. The immune system's job is a complicated process that involves the coordinated efforts of several types of white blood cells. The pictorial below depicts the process by showing how the immune system destroys viruses.
1 . The immune response begins when a white blood cell called a macrophage encounters a virus and consumes it. Meanwhile, other viruses look for nearby cells to infect.
2 . Next, the macrophage digests the virus and displays pieces of the virus called antigens on its surface. Nearby cells have now become infected by the attacking viruses.
3 . Unique among the many different helper T cells (another class of white blood cells) in the body, one particular helper T cell now recognizes the antigen displayed and binds to the macrophage.
4 . This union stimulates the production of chemical substances -- such as interleukin-1 (IL-1) and tumor necrosis factor (TNF) by the macrophage, and interleukin-2 (IL-2) and gamma interferon (IFN-y) by the T cell -- that allow intercellular communication.
5 . As part of the continuing process, IL-2 instructs other helper T's and a different class of T cells, the killer T's, to multiply. The proliferating helper T's in turn release substances that cause B cells to multiply and produce antibodies.
6 . The killer T cells now begin shooting holes in host cells that have been infected by viruses.
7 . The antibodies released by the B cells bind to antigens on the surfaces of free-floating viruses. Besides making it easier for macrophages to destroy viruses, this binding signals blood components called complement to puncture holes in the viruses.
8 . Finally, as the infection is brought under control, the activated T and B cells are turned off by suppressor T cells. However, a few "memory cells" remain behind to respond quickly if the same virus attacks again.
Cancer immunologists believe that the body fights cancer in much the same way it seeks to eliminate viruses. Further study of the immune system is expected to reveal ways to bolster it, allowing the body to become a more active partner in the fight against cancer.
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