FRCs within SLOs, such as lymph nodes, are located in the T cell zone, where they help form a fibrous network within the tissue. FRCs also produce the T cell chemoattractant CCL19, which is an important mediator of T cell trafficking. In this issue, Chung et al. present evidence that during HSCT, Ccl19-expressing stromal cells express the Notch ligands DLL1 and DLL4. DLL1/DLL4-mediated Notch signaling directly promotes the activation of alloreactive T cells, resulting in GVHD. Given their location in the T cell zone, FRCs are most likely the Ccl19-expressing cell responsible for the activation of Notch in alloreactive T cells; however, other stromal cells, including FDCs and LECs, also express Ccl19 and Notch ligands. Under normal conditions, FDCs and LECs are not likely to encounter T cells, but this possibility cannot be entirely ruled out.