Autoimmune diseases occur when the immune system mistakenly attacks which part of the body?

• A. The body's own tissues
• B. Pathogens
• C. Allergens
• D. Inert substances

Answer: (A)

Autoimmune diseases happen when the immune system loses tolerance to the body's own tissues and starts attacking them. Normally, the immune system defends you by recognizing and attacking invaders like bacteria and viruses, while leaving your own tissues alone. In autoimmunity, that self-tolerance breaks down, so the immune response is misdirected at healthy tissues such as joints, pancreas, or nerves. That’s why the best answer is the body's own tissues. Pathogens are what the immune system usually targets to defend against infections, allergens provoke reactions to harmless substances in some people, and inert substances are typically ignored by the immune system, so they don’t describe autoimmune disease.