Which system defends the body against infection and disease?

• A. Circulatory system
• B. Immune (lymphatic) system
• C. Digestive system
• D. Endocrine system

Answer: (B)

The body’s defense against infection and disease is organized by the immune (lymphatic) system. This system identifies foreign invaders like bacteria, viruses, and toxins and coordinates responses to neutralize and eliminate them. It includes white blood cells, antibodies, and organs such as lymph nodes, the spleen, thymus, and bone marrow, plus the network of lymphatic vessels that transports immune cells and fluids throughout the body. The immune system can act quickly with innate responses and adapt over time with targeted, memory-based responses, providing both immediate protection and long-term immunity.

The other systems support life in related ways but aren’t the primary defense against pathogens. The circulatory system transports blood and immune cells to sites of infection, the digestive system contributes barriers (like stomach acid and gut lining) and microbiome balance, and the endocrine system regulates hormones that influence many processes but doesn’t serve as the body’s main defense mechanism.