Reduced sensitivity of F-1 hybrid rats to re-challenge with parental strain spleen cells.
1. F1 hybrid rats that recovered from graft-versus-host (GVH) disease induced by the intraperitoneal injection of spleen cells of one parental strain showed varying degrees of resistance on re-challenge with cells of the same or of the other parental strain. Resistance to re-challenge, expressed either in terms of mortality or of severity of clinical manifestations, was greater after a strong than after a weak response to first challenge. 
 
2. Resistance was shown not to be due to immunization of the F1 hybrid. Resistant rats accepted skin grafts from both parental strains; resistance was not produced by the injection of thymic cells or of irradiated spleen cells; lethally irradiated thymectomized rats were able to develop resistance. 
 
3. Two possible explanations of this phenomenon are discussed: (a) resistance is due to reactive hyperplasia of the recipient's lymphoid system; and (b) parental lymphoid tissue contains humoral antibody-producing cells: the iso-antibodies mask antigenic sites on the recipient's target tissues from the action of cells responsible for inducing a GVH reaction, by a mechanism analogous with that of tumour enhancement.