In addition to protecting the body against infection, the immune system serves as a principal line of defense against cancer.
Therefore, it is not surprising that suppression of the immune system often results in cellular transformation and the appearance of tumors.
For example, it is not uncommon for individuals with acquired immune deficiency syndrome (AIDS) to develop a cancer called Kaposi’s sarcoma.
Tumor cells are not only functionally different from their normal predecessors but they may also be antigenically different. Tumor cells may lose the major histocompatibility antigens, may produce new antigens, or may produce antigens normally found only at much earlier (e.g., embryonic) stages of development.
Tumors that stem from infection by oncogenic viruses may display antigens that are characteristic of the viral genome. If the antigens present in the surfaces of a tumor cell are sufficiently different from those of a normal cell, the cell will be regarded as “non-self” and will be attacked by the immune system.
Natural Killer Cells:
Foreign cells that have attached antibody and virus- infected cells can be destroyed by a type of lymphocyte called a natural killer cell or NK cell (see above). The NK cell is believed to function by transferring cytotoxic substances to the target cell.
The interaction between NK cell and target cell is accompanied by the production of interferon, which serves to promote rapid proliferation of additional NK cells. In vitro studies using cultures of transformed cells indicate that NK cells can also destroy many kinds of tumor cells (e.g., leukemia, myeloma, sarcoma, and carcinoma cells).
Tumor Detection and Tumor Immunotherapy:
The growth of tumors in vivo may be modified by immunotherapy. In its simplest form, immunotherapy involves stimulating the immune system so that it may respond more vigorously to the presence of transformed cells. Immunostimulants that have met with some success are thymic hormones and doses of attenuated bacteria.
One of the most promising areas of tumor diagnosis and tumor therapy involves the use of monoclonal antibodies that have been produced against specific tumor antigens. Monoclonal antibodies that have been complexed with radioactive ligands may be injected intravenously and used to detect and localize cancerous tissue by “radioimaging.”
A most spectacular application of monoclonal antibodies for cancer therapy involves complexing tumor-specific antibodies with cytotoxic agents. These complexes would then specifically seek out and associate with tumor cells, leading to the tumor’s death. Cytotoxic agents that may be complexed with monoclonal antibodies to form such “magic bullets” include radioisotopes (whose radiation destroys the tumor cell), bacterial toxins, and plant lectins.
This form of therapy is presently in a very early stage of development but holds great promise for the future. The significant advantage of this technique over less specific chemotherapeutic measures and radiation treatment are clear.