Which structure filters lymph fluid and contains immune cells that help combat infections?

• A. Lymph vessels
• B. Bone marrow
• C. Lymph nodes
• D. Tonsils

Answer: (C)

Filtering lymph and hosting immune activity happens in the lymph nodes. As lymph moves through a node, immune cells stationed there—such as macrophages that phagocytose pathogens and lymphocytes (B cells and T cells)—survey the material for invaders. The B cells can become activated and produce antibodies, while T cells coordinate the immune response. The node’s architecture supports this defense: the cortex is rich in B cells and germinal centers for antibody maturation, the paracortex houses T cells, and the medullary area contains additional macrophages and plasma cells. This combination of filtration and immune-ready cells is what makes a lymph node fit the description.

Lymph vessels mainly serve as the pathways that transport lymph; they aren’t filtration hubs with concentrated immune activity. Bone marrow is the production site for immune cells rather than a filter for lymph. Tonsils are lymphoid tissue that help guard against pathogens at entry points (mouth and throat), but they don’t filter circulating lymph in the same way as lymph nodes.