Page 1158 - Veterinary Immunology, 10th Edition

VetBooks.ir Failure of Immunity to Cancer Cells

The fact that tumors are so readily induced and are so relatively
common testifies to the inadequacies of the immunological

protective mechanisms. Studies of tumor-bearing animals have
indicated several mechanisms by which immune systems fail to
reject tumors.

Cancer Cell Selection

Cells do not usually become malignant in a single step. Rather, they
gradually become malignant over a long period, going from benign
to malignant in a process called tumor progression. The process
occurs through a series of mutations that switch genes on and off.

These mutations do not necessarily alter the immunogenicity of
cancer cells, or do so in small steps. Immunogenicity may not alter
until the cells are irreversibly committed to malignancy. Thus there
are two selection mechanisms by which cancer cells can evade the

host's immune response and so enhance their own survival. One is
“sneaking through,” the process by which malignant cells may not
trigger an immune response until the tumor has reached a size at
which it cannot be controlled by the host. Thus in experimental

tumors, small numbers of cancer cells may grow after subcutaneous
inoculation, although large numbers may not. It may be that the
cancer cells may not reach lymph nodes and trigger an immune
response until the tumor burden is too large to be controlled. Even

a very small tumor may contain an enormous number of cells. For
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example, a 10-mm tumor contains about 10 cells. The second
mechanism, cancer cell selection, reflects the fact that cancer cells
that have mutated in such a way as to be antigenically different

from the host will induce a strong immune response and be
eliminated without leading to disease. Cancer cells survive because
of their lack of antigenicity and their inability to stimulate an
immune response. To this extent, therefore, tumors that do develop

have, by definition, already beaten the immune system.

Immunosuppression

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