CD40 and infection: Although lacking mature tertiary lymphoid structures, known lung TFH cells, or dependence on the follicular chemokine CXCL13, we observed proliferative B2 and GC B cells in situ in the lung after two Sp19F infections, suggesting that follicular signaling such as CD40-CD40L interactions between T and B cells may take place locally after the second exposure to Sp19F (38, 55).