FCGRT and graft versus host disease: There are 2 principal and not mutually exclusive pathways that may account for the accelerated loss of humoral immunity after BMT: (a) loss of IgG from mucosal sites during GVHD (i.e., protein-losing enteropathy) (39) and (b) enhanced serum IgG catabolism due to defects in the expression of FcRn, which is the Fc receptor responsible for IgG recycling in vivo (15).