Model for gut microbiota mediation of chronic GVHD. Early posttransplant IgA antibody response to commensal bacteria in the colon leads to clonal expansion of plasmablasts, associated with a higher risk for future cGVHD. The precise mechanism by which the process of bacterial recognition (and possibly immune-mediated clearance), IgA production, and clonal plasmablast expansion may lead to cGVHD is unknown.