Tumor Immunology

Tumor cells can reveal antigens that stimulate the destruction of the tumor. When cancers develop, this immunological surveillance system—primarily the function of T cells and natural killer cells—has failed to prevent the growth and metastasis of the tumor

Oncology (the study of tumors) has revealed that tumor biology is similar to and interrelated with the functions of the immune system. Most tumors appear to be clones of single cells that have become transformed in a process similar to the development of lymphocyte clones in response to specific antigens. Lymphocyte clones, however, are under complex inhibitory control systems—such as those exerted by suppressor

T lymphocytes and negative feedback by antibodies. The division of tumor cells, by contrast, is not effectively controlled by normal inhibitory mechanisms. Tumor cells are also relatively unspecialized—they dedifferentiate, which means that they become similar to the less specialized cells of an embryo.

Tumors are described as benign when they are relatively slow growing and limited to a specific location (warts, for example). Malignant tumors grow more rapidly and undergo metastasis, a term that refers to the dispersion of tumor cells and the resultant seeding of new tumors in different locations. The term cancer, as it is generally applied, refers to malignant tumors.

As tumor cells dedifferentiate, they reveal surface antigens that can stimulate the immune destruction of the tumor. Consistent with the concept of dedifferentiation, some of these antigens are proteins produced in embryonic or fetal life and not normally produced postnatally. Since they are absent at the time immunological competence is established, they are treated as foreign and fit subjects for immunological attack when they are produced by cancerous cells. The release of two such antigens into the blood has provided the basis for laboratory diagnosis of some cancers. Carcinoembryonic antigen tests are useful in the diagnosis of colon cancer, for example, and tests for alpha-fetoprotein (normally produced only by the fetal liver) help in the diagnosis of liver cancer.

Tumor antigens activate the immune system, initiating an attack primarily by killer T lymphocytes (fig. 15.25) and natural killer cells (described in the next section). The concept of immunological surveillance against cancer was introduced in the early 1970s to describe the proposed role of the immune system in fighting cancer. According to this concept, tumor cells frequently appear in the body but are normally recognized and destroyed by the immune system

Tumor Immune System

■ Figure 15.25 T cell destruction of a cancer cell. A killer T cell (a) contacts a cancer cell (the larger cell), in a manner that requires specific interaction with antigens on the cancer cell. The killer T cell releases lymphokines, including toxins that cause the death of the cancer cell, as shown in (b).

Scanning electron micrographs © Andrejs Liepens.

■ Figure 15.25 T cell destruction of a cancer cell. A killer T cell (a) contacts a cancer cell (the larger cell), in a manner that requires specific interaction with antigens on the cancer cell. The killer T cell releases lymphokines, including toxins that cause the death of the cancer cell, as shown in (b).

Scanning electron micrographs © Andrejs Liepens.

The Immune System 469

before they can cause cancer. There is evidence that immunological surveillance does prevent some types of cancer; this explains why, for example, people with AIDS (who have a depressed immune system) have a high incidence of Kaposi's sarcoma. It is not clear, however, why all types of cancers do not appear with high frequency in AIDS patients and others whose immune systems are suppressed. For these reasons, the generality of the immunological surveillance system concept is currently controversial.

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