On binding antigen on the B cell through its T-cell receptor, the helper T cell is induced to express CD40 ligand (CD40L), which binds to CD40 on the B cell. As shown in the top left panel, the tight junction formed between the cells upon antigen-specific binding seems to be sealed by a ring of adhesion molecules, with LFA-1 on the T cell interacting with ICAM-1 on the B cell (see Fig. 8.30). The cytoskeleton becomes polarized, as revealed by the relocation of the cytoskeletal protein talin (stained red in right center panel), to the point of cell-cell contact, and the secretory apparatus (the Golgi apparatus) is reoriented by the cyto-skeleton toward the point of contact with the B cell. As shown in the bottom panels, cytokines are released at the point of contact. The bottom right panel shows IL-4 (stained green) confined to the space between the B cell and the helper T cell. MTOC, microtubuleorganizing center. Photographs courtesy of A. Kupfer.