Page 1178 - Veterinary Immunology, 10th Edition

VetBooks.ir Lymphoid Tumors

Adaptive immunity requires that antigen-sensitive cells stimulated
by exposure to antigen respond by division and differentiation.

Much of the complexity of the immune system is due to the need to
control this response. A failure may result in uncontrolled
lymphoid cell proliferation and the development of lymphoid
tumors. The surveillance theory was originally proposed when it
was observed that immunosuppressed animals and humans had an

increased prevalence of cancers. However, an unusually high
proportion of these are of lymphoid origin. Therefore it is likely that
some of the lymphoid neoplasia that develop in

immunosuppressed individuals result from a failure of
immunological regulation rather than from a failure of surveillance.
Normal immune responses, whether antibody or cell mediated,
involve a burst of rapid proliferation in lymphocytes. This burst of
proliferation must be carefully controlled (Chapter 18). Although

uncontrolled lymphocyte function may induce autoimmunity,
uncontrolled lymphocyte proliferation may result in the
development of a lymphoma or lymphosarcoma. It is no accident

that animals with autoimmune disease are more likely than normal
animals to develop lymphoid cell tumors (Table 35.1).

TABLE 35.1

Immunosuppressive Effects of Lymphoid Tumors

Cell
Tumor Evidence for Immunosuppression Mechanisms
Type
Feline leukemia T cell Lymphopenia Suppressive viral protein,
Prolonged skin grafts pI5E
Increased susceptibility to infection Suppressor cells
Lack of response to mitogens
Marek's disease T cell Lack of response to mitogens Suppressor macrophages
Depressed cell-mediated cytotoxicity
Depressed IgG production
Avian lymphoid leucosis B cell Increased susceptibility to infection Suppressor lymphocytes
Bovine leucosis B cell Depressed serum IgM Soluble suppressor factor
Myeloma B cell Increased susceptibility to infection Soluble tumor-cell factor
Negative feedback
Canine malignant B cell Predisposition to infection associated with autoimmune Unknown
lymphoma disorders
Equine lymphosarcoma T cell Increased susceptibility to infection Tumor of suppressor cells
Several important viruses stimulate nonspecific lymphocyte

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