Cancer occurs when cells grow out of control. This can happen with the cells of the immune system. The tumor cells may not perform normal functions, which can lead to immunodeficiencies. Alternatively, tumor cells can be functional and produce large numbers of antibodies.
The normal immune system requires a rapid increase in the growth of lymphocytes to fight foreign invaders. On occasion, this increase in the growth of lymphocytes may be uncontrolled, which causes a tumor called lymphoma. Lymphoma occurs in middle-aged to older cats. No breed of cat is known to have a higher risk for lymphoma than other breeds. Infections with both feline leukemia virus and feline immunodeficiency virus have been shown to increase the risk for developing lymphoma.
Because lymphocytes are present in all organs, lymphoma can develop in any organ, especially the lymph nodes, chest, digestive tract, kidneys, nervous system, eyes, skin, nose, and blood. Signs in cats with lymphoma depend primarily on the location of the tumor cells, but can include enlargement of lymph nodes, vomiting, diarrhea, loss of appetite, weight loss, lethargy, difficulty breathing, and increased thirst or urination. The skin form can cause redness or flakiness of the skin, ulceration (especially near the lips and on the footpads), itching, or lumps in the skin. The disease is diagnosed by blood tests and biopsy of a lump or enlarged lymph node. It is often treated with chemotherapy involving several drugs. Adverse effects of the chemotherapy include vomiting, diarrhea, lack of appetite, and fever. Hair loss as an effect of chemotherapy does not occur in cats.
Also see professional content regarding immune system tumors.