Page 1149 - Veterinary Immunology, 10th Edition

Most humans who develop “spontaneous” cancer have a normal
VetBooks.ir immune system. Immunosuppressed individuals such as allograft

recipients and AIDS patients develop a different spectrum of
cancers from that of the general population. The only cancers to

which they are at greater risk are lymphoid tumors and those
caused by viruses, such as Kaposi's sarcoma. Immunosuppressed
humans are at no more likely than the general population to
develop the common cancers such as those of breast, lung, or colon.

The situation is the same in animals. For example, in a study of 111
cats that received kidney allografts, 25 of these developed cancer.
The most common tumor was a lymphoma that developed in 14
cats. All the lymphomas that were characterized were mid- to high-

grade large B cell lymphomas. This is also the most common type of
tumor in human transplant recipients.

Tumor Neoantigens

Cancer cells develop as a result of multiple mutations. These
mutations may generate molecules that are unique to the cancer
cells (tumor-specific antigens) or, more commonly, abnormal or
unusual molecules (tumor-associated antigens). To distinguish
between normal and cancer cells, host T cells must recognize these

new antigens. Five major types of tumor antigen have been
identified. First, there are differentiation antigens associated with
specific stages in the development of a cell type. For example, some

cancer cells express the products of developmental genes that are
turned off in adult cells and are only expressed early in an
individual's development. These proteins are called oncofetal
antigens. Examples include tumors of the gastrointestinal tract that
produce a glycoprotein called carcinoembryonic antigen (CEA; also

called CD66e), normally found only in the fetal intestine. The
presence of CEA in serum may indicate the presence of a colon or
rectal adenocarcinoma. α-fetoprotein produced by hepatoma cells is

normally found only in the fetal liver. Likewise, squamous cell
carcinoma cells may possess antigens normally restricted to fetal
liver and skin. These oncofetal antigens are usually poor
immunogens and do not provoke protective immunity. However,
their detection may be useful for diagnosis and for monitoring the

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