The sequence of events that leads to the development of a T-dependent B-cell response has been studied for many years and much attention has focused on the importance of these interactions for the B cell and the antibody response. While a role for B cells in T-cell memory has been demonstrated, the cellular and molecular mechanisms underlying this are unclear and the importance for T cells to migrate into follicles and interact with B cells has received relatively little attention. These interactions between T and B lymphocytes are facilitated by a co-ordinated series of migration events within lymph nodes and here we propose that this migration and these events are essential for the terminal differentiation of all CD4+ T cells and without it tolerance may result.