We chose peritoneal cells, depleted of most T cells (to avoid GVHD), as a source of donor lymphocytes because they contain a large population of self-sustaining lymphocytes, the B-1 cells (C19+CD43+CD5+) [23], and because of the tolerance observed in recipients with long lasting passenger B cell chimerism (Figure 2H). This evidence concerns the gene SPN and graft versus host disease.