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

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

Answer: (A)

The immune system is the body’s defense that recognizes and attacks foreign substances. It includes immune cells such as white blood cells (like phagocytes and lymphocytes) and immune organs like lymph nodes, the spleen, thymus, and bone marrow. When a foreign substance or pathogen is detected, innate defenses respond quickly with general barriers, while adaptive defenses target specific invaders and create memory for faster responses in the future. Antibodies produced by B cells tag invaders for destruction, and T cells can destroy infected cells or coordinate the immune response. Other body systems help support this defense—nervous signals, circulation of immune cells, and digestion-related barriers and nutrients—but the primary role of recognizing and attacking foreign substances lies with the immune system.