Autoimmune disease is characterized by the immune system reacting against which of the following?

• A. Foreign pathogens
• B. Own constituents
• C. Lymphoid tissues only
• D. All nonself antigens

Answer: (B)

Autoimmune disease occurs when the immune system turns against the body’s own tissues because self-tolerance has been lost. Normally, the immune system learns to ignore self-antigens, but when that tolerance breaks down, autoreactive cells and autoantibodies target the body’s own constituents, leading to inflammation and tissue damage across various organs. This is why the correct description is the immune system reacting against own constituents. Attacking foreign pathogens would be a response to invaders, not autoimmunity; targeting lymphoid tissues only isn’t what defines autoimmune disease; and attacking all nonself antigens would imply a broad, non-specific reaction, not the self-directed focus characteristic of autoimmunity.