Understanding The Immune System

But a Fetus Is Not Rejected A fetus, which carries foreign antigens: from its father as well as immunologically compatible self antigens from its mother, might be expected to trigger a graft rejection. But the uterus is an "immunologically privileged" site where immune responses are subdued. One source of protection appears to be a substance produced by the fetus, perhaps in response to antibodies from the mother: the substance promotes the development of special white blood cells in the uterus, and these cells release a factor that blocks the actions of IL-2. Another substance, produced by the uterus, helps disguise antigens on the fetal surface of the placenta, shielding them from the mother's immune defenses.

Immunity and Cancer The immune system provides the body's main defense against cancer. When normal cells turn into cancer cells, some of the antigens on their surface change. These new or altered antigens flag immune defenders, including cytotoxic T cells, natural killer cells, and macrophages. According to one theory, patrolling cells of the immune system provide continuing bodywide surveillance, spying out and eliminating cells that undergo malignant transformation. Tumors develop when the surveillance system breaks down or is overwhelmed. Some tumors may elude the immune defenses by hiding or disguising their tumor antigens. Alternatively tumors may survive by encouraging the production of suppressor T cells; these T cells act as the tumor's allies, blocking cytotoxic T cells that would normally attack it. Blood tests show that people can develop antibodies to many types of tumor antigens (although the antibodies may not actually be effective in fighting the tumor). Skin testing (similar to skin testing for tuberculosis) has demonstrated that tumors provoke cellular immunity as well. Furthermore, studies indicate that cancer patients have a better prognosis when their tumors are infiltrated with many immune cells. Immune responses may underlie the spontaneous disappearance of some cancers. Tests using antibodies derived from batches of human serum can detect various tumor-associated antigens-including carcinoembryonic antigen (CEA) and alphafetoprotein (AFP)-in blood samples. Because such antigens develop not only in cancer but in other diseases as well, the antibody tests are not useful for cancer screening in the general population. They are, however, valuable in monitoring the course of disease and the effectiveness of treatment in patients known to have cancer. More recently, scientists have developed monoclonal antibodies (see Hybridoma Technology) that are targeted specifically at tumor antigens. Linked to radioactive substances, these antibodies can be used to track down and reveal hidden cancer metastases within the body Monoclonal antitumor antibodies are also being used experimentally to treat cancer-either in their native form or as immunotoxins, linked to anticancer drugs or radioactive substances. Other efforts to attack cancer through the immune system center on stimulating or replenishing the patient's immune responses with substances known as biological response