Alterations in cytokine levels including IL-10, IL-6, and IL-2 via immunoregulatory single nucleotide polymorphisms (SNPs) can lead to altered immunoregulatory function of regulatory T (Treg) and effector T (Teff) cells. Binding of CTLA4 with its receptor (possibly also via functional SNPs) with CD80/CD86, B7, proteins on dendritic cells can lead to induction of indolamine 2,3 dioxygenase (IDO) and the catabolism of tryptophan into proapoptotic metabolites causing immunosuppression of Teffs. Altered binding of CTLA4 conversely may lead to reduced immunosuppression via Tregs and GVHD. High IL-6 levels induced in DCs by Treg interaction can also cause alteration of Tregs to Th17 cells and may lead to or exacerbate GVHD. Professional illustration by Alice Y. Chen.