How does the immune system work to avert cancer and other diseases?
Although scientists have more to learn about the interplay between the immune system and cancer, much is already known about the immune response. The immune system scans the body to identify any substance -- natural or synthetic, living or inert -- that it considers foreign. In other words, it distinguishes "self" from "nonself" when it detects an invading organism. The immune system consists of several different types of white blood cells, located throughout the body, that work together in a highly integrated way.
The immune response begins when a white blood cell called a macrophage encounters a nonself organism, perhaps a bacterium or a virus. The macrophage literally eats the substance, digests it and then displays certain pieces of the invader, called antigens, on its surface. These antigenic fragments alert a specific type of T lymphocyte, the helper T cell, to begin a precisely choreographed attack against the foreign intruder. The helper T cell that is called to action is unique in its ability to respond to the antigen displayed. Incredibly, the human body produces countless T cells, each programmed to detect a specific nonself antigen.
The helper T cell begins the attack by binding to the macrophage via the T cell's antigen receptor. This union, aided by a third cell (dendritic cell) that makes immune cells cluster, stimulates both the macrophage and the helper T cell to exchange chemical messages between themselves and with other cells of the immune system. The macrophage releases cytokines called interleukin-1 (IL-1) and TNF, or tumor necrosis factor. TNF steps up the production of IL-1 and performs many of the same functions that IL-1 does, including inducing fever to help the body combat infection more effectively. IL-1 alone enhances the ability of dendritic cells to form immune cell clusters and stimulates the helper T cell to release several lymphokines, one of which is called interleukin-2 (IL-2). IL-2 in turn causes T cells to secrete gamma interferon, which, among other things, activates macrophages. IL-2 also instructs other helper T cells and a different class of T cells, the killer T cells, to multiply.
By the time this multiplication process gets under way, the invader will have started to multiply as well and some of the resulting entities will have been consumed by other macrophages. Many of the "daughter invaders" will have escaped, however, and some of these will have started infecting host cells. At this point, the killer T cells begin shooting holes in the infected host cells. Simultaneously, the proliferating helper T cells release substances that signal another type of lymphocyte, the B cell, to begin multiplying and differenting into antibody-producing cells.
The antibodies released by the B cells bind in a lock-and-key fashion to antigens on the surfaces of invaders that have escaped macrophages. This interaction makes it easier for the macrophages and special killer lymphocytes, called natural killer cells, to destroy these unwelcome entities. The binding of the antibodies to the foreign antigens also signals a group of blood components, called complement, to puncture the invaders' membranes, leading to their death.
Finally, as the infection is brought under control, yet another type of T cell, the suppressor T cell, tells the B cells, the helper T cells and the killer T cells to turn off. Most of these immune cells will die, but a few will remain in the body. These cells, called memory cells, will be able to respond more quickly the next time the body is invaded by the same foreign substance that activated them this time.
The foregoing description of the immune response applies mainly to viruses and bacteria. The immune system is believed to behave similarly, however, when it encounters cancer cells, which it also recognizes as foreign and then destroys. In laboratories, scientists have observed killer T cells, natural killer cells, macrophages and TNF destroying malignant tumor cells. They have found that certain antibodies that recognize tumor cells help the macrophages and the natural killer cells accomplish their mission. Further study of the immune system is expected to reveal additional evidence that the body defends itself against cancer in much the same way that it seeks to eliminate other intruders
