Which system defends against infection?

• A. Respiratory System
• B. Circulatory System
• C. Digestive System
• D. Immune System

Answer: (D)

Defending against infection relies on the immune system, which orchestrates both nonspecific and specific defenses. Innate defenses act quickly with barriers (like skin and mucous membranes), phagocytes, and inflammation to keep pathogens out and contained. If invaders slip past, adaptive defenses recruit T cells and B cells to target specific pathogens, producing antibodies and creating memory for faster future responses. Other systems support this process—airways use mucus and cilia to trap invaders, the digestive system uses acid and gut flora to limit microbes, and the circulatory system transports immune cells and antibodies—but the immune system is the one dedicated to defending against infection.