The immune system consists of a network of white blood cells, antibodies, and other substances that fight off infections and reject foreign proteins. In addition, the immune system includes several organs. Some, such as the thymus gland and the bone marrow, are the sites where white blood cells are produced. Others, including the spleen, lymph nodes, and liver, trap microorganisms and foreign substances and provide a place for immune system cells to collect, interact with each other and with foreign substances, and generate an immune response.

The primary role of the immune system is to defend the body against foreign invaders or abnormal cells that invade or attack it. The immune system functions in the same way in horses as it does in dogs—and, indeed, in humans. For that reason, the various immune system responses to foreign substances and the types of immune system disorders are discussed in detail in the chapter on immune disorders in dogs see Immune Disorders of Dogs: The Immune System of Dogs.

Last full review/revision July 2011 by Christine Andreoni; Kevin T. Schultz, DVM, PhD