Intestinal damage and dysbiosis linked to GVHD. Upon HCT, multiple factors such as conditioning toxicity, antibiotic treatment, and immune activation mediate GVHD. GVHD progression associates with injury to stem cell compartments along with Paneth cells (small intestine) and goblet cells. This leads to increased intestinal permeability, inflammation, and reduction of the mucus layer and antimicrobial products (eg, defensins). Antibiotic treatment and a limited amount of nutrients in the gut also promote gut dysbiosis, furthering gastrointestinal damage and disease. DC, dendritic cell; IgA, immunoglobulin A.