Which fluid in the lymphatic system helps defend against infections?

• A. Plasma
• B. Lymph
• C. Serum
• D. Platelets

Answer: (B)

The fluid that helps defend against infections in the lymphatic system is lymph. Lymph is formed from interstitial fluid and travels through lymphatic vessels, carrying immune cells such as lymphocytes to lymph nodes and other lymphoid tissues where pathogens are detected and immune responses are coordinated. This transport and filtration in the lymphatic system is how the body monitors for and fights infections.

In contrast, plasma is the liquid component of blood that circulates within vessels and carries nutrients, hormones, and antibodies; serum is plasma minus clotting factors; and platelets are cell fragments involved in blood clotting. While antibodies can be present in plasma, the specific immune defense activity tied to the lymphatic system—immune surveillance and response via lymph carrying immune cells—happens through lymph.