Which system includes cells and tissues that recognize and attack foreign substances in the body?

• A. Nervous system
• B. Circulatory system
• C. Endocrine system
• D. Immune system

Answer: (D)

The system that recognizes and attacks foreign substances in the body is the immune system. It includes cells such as white blood cells—like lymphocytes (B cells and T cells) and macrophages—and tissues such as lymph nodes, the spleen, and the thymus. These components work together to identify invaders, such as bacteria, viruses, and toxins, by recognizing foreign molecules called antigens. The immune system uses different strategies: innate defenses provide general protection right away, while adaptive defenses tailor a targeted response to specific invaders and can remember them for faster future protection. Antibodies produced by B cells can neutralize pathogens, and T cells can destroy infected cells or help coordinate the response. Although the circulatory, nervous, and endocrine systems interact with the immune system, they are not the systems whose primary job is recognizing and attacking foreign substances.