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Disorders Involving Cytotoxic Antibodies (Type II Reactions) in Cats


Ian Rodney Tizard

, BVMS, BSc, PhD, DSc (Hons), DACVM, Department of Veterinary Pathobiology, College of Veterinary and Biomedical Sciences, Texas A&M University

Reviewed/Revised Aug 2018 | Modified Oct 2022

Type II reactions occur when an antibody binds to an antigen present at the surface of its own cells. These reactions can lead to several types of diseases in cats, including anemia, blood clotting problems, and skin and muscle disorders. They may be associated with other immune system disturbances, such as cancers of the lymphoreticular system, or triggered by a drug, vaccine, or infection. Most often, the triggering cause cannot be pinpointed. Immune-mediated hemolytic anemia and thrombocytopenia are the most common Type II reactions.

Immune-mediated Hemolytic Anemia

Anemia occurs when an animal has an abnormally low number of red blood cells. Immune-mediated hemolytic anemia is a severe and life-threatening disease in which the cat’s immune system sees its own red blood cells as foreign invaders, and therefore produces antibodies to destroy them. Red blood cells are manufactured as usual in the bone marrow, but once released into the bloodstream, they are attacked and destroyed by antibodies. Signs of anemia may include fatigue, paleness of the lips and gums, and depression, along with jaundice in some cases. Other signs your veterinarian may find include an enlarged liver or spleen.

Immune-mediated hemolytic anemia has 4 basic forms, peracute, acute or subacute, chronic, and pure red cell aplasia. The chronic (longterm) form is most common in cats. Most forms are treatable with drugs, including corticosteroids and cytotoxic drugs, such as those often used in chemotherapy. Blood transfusions might be necessary. Relapses are common.

Immune-mediated Thrombocytopenia

Thrombocytopenia occurs when an animal has an abnormally low number of platelets, which are responsible for blood clotting. When an animal has thrombocytopenia, clotting does not occur correctly. Even minor injuries can cause uncontrollable bleeding, resulting in the loss of red blood cells (anemia). The most frequent signs are bleeding and bruising of the skin and mucous membranes. Abnormal bleeding into the feces or urine as well as nosebleeds can occur. Immune-mediated thrombocytopenia is caused by the destruction of platelets (thrombocytes) by the immune system in much the same manner as red blood cells are destroyed in immune-mediated hemolytic anemia (see above).

Before immune-mediated thrombocytopenia can be diagnosed, many more common diseases must be excluded, including various clotting disorders, infections or cancer, and intestinal parasites. The diagnosis is usually made based on signs and response to treatment, rather than on blood tests. However, certain tests, such as platelet counts, clotting profiles, and bone marrow sampling are helpful. Medications that suppress the immune system will likely be prescribed to treat this disease. Signs usually disappear after 5 to 7 days of treatment when platelet counts begin to rise. If the platelet count has not increased significantly after 7 to 10 days, additional or different medications may be prescribed. If the blood loss is life threatening, transfusions of whole blood or plasma may be necessary.

Treatment is often continued for 1 to 3 months after the platelet counts return to normal. Some cats have persistent decreases in platelets even with drug treatment. If this is the case with your pet, you and your veterinarian will want to discuss longterm treatment and maintenance options.


Pemphigus foliaceus is an uncommon autoimmune disease that affects the skin. It is rare in cats. The cause of this malfunction of the immune system is usually not known. One theory is that the skin is somehow altered, making it appear “foreign” to the immune system. In the case of pemphigus, the immune system produces antibodies against the “glue” that normally keeps skin cells (keratinocytes) attached to one another. White blood cells move in causing further damage, and the keratinocytes break apart from each other, forming pimples or crusted areas. Veterinarians frequently prescribe corticosteroids for initial treatment of pemphigus, but other immunosuppressive drugs may be added if there is no response.

Myasthenia Gravis

Myasthenia gravis is an autoimmune neuromuscular disease; it is rarely seen in cats. Weakness, especially with exercise, and lack of muscle control are the primary signs. Affected animals produce antibodies to certain nerve receptors and destroy them. This leads to the inability to contract muscles and extreme muscle weakness. Myasthenia gravis is diagnosed using blood tests to search for the antibody to the nerve receptor and other tests to measure nerve activity. Your veterinarian will prescribe drugs to control this disease. Remissions can occur, and many animals that develop this condition do well with continued treatment.

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