Damage caused by the pretransplant conditioning regimen allows luminal bacteria to invade the ileal mucosa. Recipient neutrophils attracted as part of the inflammatory response infiltrate the ileal mucosa and upregulate expression of major histocompatibility complex (MHC) class II molecules. Activated neutrophils then migrate to draining lymph nodes where they present recipient alloantigens to donor T cells. Activated donor T cells subsequently traffic to target tissues and cause GVHD. Professional illustration by Somersault18:24.