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). Here, FCGRT is linked to Protein-losing enteropathy.