B cell receptors recognize free (soluble) antigen primarily in which locations?

• A. Lungs
• B. Blood or lymph
• C. Bone marrow
• D. Lymphoid follicles only

Answer: (B)

B cell receptors are membrane-bound immunoglobulins on naive B cells that survey soluble antigens in the extracellular fluids. Free, soluble antigens travel through the blood and are carried via the lymphatic system, so naive B cells constantly circulate between blood and lymph and encounter these antigens as they pass through secondary lymphoid organs like lymph nodes and the spleen. This makes blood and lymph the primary arenas where BCRs recognize soluble antigens, rather than the bone marrow (where developing B cells are educated), the lungs, or exclusively within lymphoid follicles. In essence, the fluids that transport antigen—blood and lymph—are where BCRs most often first bind free antigen.