Hypothetical model of the relationship between intestinal bacterial diversity and GVHD. (A) When bacterial diversity is high, antimicrobial peptides such as Reg3α can protect intestinal epithelial cells from damage and GVHD is averted. (B) When bacterial diversity is low, Reg3α is insufficient to counter the overwhelming number of bacteria resulting in intestinal epithelial cell apoptosis and GVHD.