FCs matched to HSCs at class I K enable engraftment in allogeneic recipients. HSCs (5000) and FCs (30 000) were sorted from donors disparate at the indicated MHC loci, mixed, and transplanted into allogeneic B10 recipients. (A) Shading shows the MHC disparity between FC donor and HSC donor. (B) Survival of recipients of HSCs plus FCs or FCs alone from B10.BR. (C) Survival of recipients given B10.BR HSCs (H-2^k) plus FCs from B10.A(4R) donors (MHC disparate to HSCs at class I D and L and also lacking class II I-E expression), from B10.MBR donors (MHC disparate to HSCs at class I K, D, and L), and from H-2 disparate C57BL/10 (H-2^b) donors. (D) MHC disparity between HSC and FC donors used in experiments shown in panel E. (E) Survival curve of B10 recipients given HSCs and FCs that were mismatched only at class I K (B10.AKM HSCs plus B10.MBR FCs).