We propose at least three factors as important for determining the sites of relapse in MS patients: (1) the existence of excess MHC class II+CD11b+ cells at a particular site, (2) activation of a sympathetic pathway that accumulates MHC class II+CD11b+ cells in certain blood vessels, and (3) the presence of organ-specific autoreactive CD4+ T cells in the periphery. This evidence concerns the gene CD4 and myeloid sarcoma.