Lymph nodes and spleen play what roles in immunity?

• A. Lymph nodes filter lymph and mount immune responses; the spleen filters blood, removes old red blood cells, stores platelets, and participates in immune responses.
• B. Lymph nodes filter blood; the spleen filters lymph.
• C. Both organs filter plasma and do not participate in immune responses.
• D. Lymph nodes store platelets; spleen filters bile.

Answer: (A)

Lymph nodes and the spleen are key sites where the immune system encounters foreign invaders and mounts responses, but they handle different fluids. Lymph nodes act as filters for lymph, the fluid that drains from tissues. As lymph flows through, antigen-presenting cells bring in pathogens and present them to B and T cells, triggering activation, clonal expansion, and the production of antibodies or effector responses. This is how the body mounts targeted, adaptive immunity to pathogens that travel through the lymphatic system.

The spleen handles blood. Its white pulp contains immune tissue that scans the bloodstream for antigens and initiates immune responses against blood-borne pathogens. The red pulp, meanwhile, filters the blood by removing old or damaged red blood cells and works with macrophages to break down pathogens. The spleen also stores platelets and contributes to rapid immune reactions to blood-borne threats.

So the combined roles fit: lymph nodes filter lymph and mount immune responses; the spleen filters blood, removes old red blood cells, stores platelets, and participates in immune responses.