Which structures filter lymph and house immune cells?

• A. Lymph nodes
• B. Spleen
• C. Thymus
• D. Tonsils

Answer: (A)

Lymph nodes serve as the checkpoints of the lymphatic system. They filter lymph as it travels through the vessels, using resident macrophages and dendritic cells in the subcapsular and medullary sinuses to trap and phagocytose pathogens. At the same time, they house immune cells—B cells in the cortex and T cells in the paracortex—so antigens encountered in the lymph can be presented to lymphocytes, triggering an immune response. This combination of filtration and immune surveillance is why lymph nodes are the structures that filter lymph and house immune cells.

The spleen filters blood, not lymph. The thymus is the site where T cells mature, not where lymph is filtered. Tonsils monitor antigens at mucosal surfaces rather than filtering circulating lymph.