Where do lymphocytes mainly contact antigens?

• A. Bloodstream
• B. Thymus
• C. Bone marrow
• D. Secondary lymphoid organs

Answer: (D)

Lymphocytes mainly contact antigens in secondary lymphoid organs, where organized microenvironments and antigen-presenting cells enable activation. Naive lymphocytes constantly recirculate through these sites—lymph nodes, spleen, tonsils, and mucosa-associated lymphoid tissue—where they can meet antigens carried by dendritic cells, macrophages, or B cells.

In peripheral tissues, antigen-presenting cells sample materials and migrate via lymphatics to draining lymph nodes, presenting antigens to T cells in the T-cell zones. B cells encounter antigens in the B-cell follicles and germinal centers, especially as antigens arrive through lymph or bloodstream. The spleen’s white pulp handles blood-borne antigens in a similar way, with distinct areas for T and B cell interactions.

Primary lymphoid organs like the thymus and bone marrow are where lymphocytes develop and mature, not where they primarily meet antigens. So the main contact occurs in secondary lymphoid organs.