What statement best describes the main characteristic of autoimmune disease?

• A. The animal's immune system is abnormally aggressive against antigens.
• B. The animal's immune system lacks some of the cells that contribute to the immune response.
• C. The animal's immune system reacts against the animal's own constituents.
• D. The animal's immune system only reacts against some types of antigens.

Answer: (C)

Autoimmune disease occurs when the immune system turns against the body's own tissues. Normally the immune system learns to ignore self-components, but in autoimmunity this tolerance is lost, and autoantibodies or autoreactive T cells target the animal’s own organs and cells. This self-directed attack can be systemic or organ-specific and explains why the condition can cause a wide range of symptoms depending on which tissues are affected.

This description fits best because it captures the essence of autoimmunity: a immune response directed at self constituents rather than just responding aggressively to external antigens. It’s different from scenarios where the immune system is simply overactive in general or missing parts of the immune system (which would weaken defense) or where responses are limited to only certain external antigens.