The cells of the immune system may become neoplastic. This results in the production of tumor cells that may be totally ineffective and hence lead to immunodeficiencies. Alternatively they may be functional and produce large quantities of immunoglobulins against unknown antigens.

In cancer patients, tumor cells escape from the immune attack by relying on both immunosuppression and tumor cell modification. The demonstration that even bulky, invasive tumors can undergo complete remission under appropriate stimulation (eg, IL-2) has shown that it is indeed possible to treat cancer successfully by immune manipulation.

Lymphomas are one of the most prevalent tumors in dogs and cats. The normal immune response requires a burst of rapid proliferation of lymphocytes. On occasion however, this proliferation may be uncontrolled, and lymphoid neoplasms result. Because lymphocytes are present in all organs, tumor development can occur in any organ. Lymphomas can be multicentric, mediastinal, gastrointestinal, renal, nervous, or leukemic. Less commonly, they occur in the eyes, skin, or nose. To determine the stage of the disease, CBC, serum chemistry profiles, abdominal ultrasound, abdominal radiographs, and bone marrow analyses are useful. Immuno-fluorescent staining can be performed in dogs and cats to characterize lymphomas. They may be either T cell or B cell in origin.

Most cases of canine lymphosarcoma, Marek's disease, calf leukosis, and feline leukemia are of T-cell origin, as are thymomas. Many T-cell lymphomas are associated with a simultaneous immunosuppression manifest by a predisposition to recurrent infections.

Adult bovine and ovine leukosis, alimentary feline leukemia, and avian leukosis are usually of B-cell origin. Under some circumstances, neoplastic B cells may develop into plasma cells. Plasma-cell tumors are known as myelomas. Because neoplastic plasma cells can secrete immunoglobulins, they give rise to gammopathies (see below).

Last full review/revision July 2011 by Ian Tizard, BVMS, PhD, DACVM