What is the primary function of the lymphatic system?

• A. Drain urine from kidneys
• B. Produce red blood cells
• C. Filter air in lungs
• D. Return interstitial fluid to the bloodstream and support immune defense

Answer: (D)

The lymphatic system’s main job is to manage fluid balance and support immune defense. In tissues, plasma filters out and becomes interstitial fluid; most of this fluid is reabsorbed into blood vessels, but a portion enters lymphatic vessels and is eventually returned to the bloodstream. This return helps keep blood volume stable and prevents edema. While the fluid travels, lymph passes through lymph nodes and other lymphoid organs where immune cells scan for pathogens and initiate immune responses. These functions together—reclaiming interstitial fluid and providing immune surveillance—are what the lymphatic system primarily does.