After stem cell transplantation, the donated immune cells sometimes attack the patients‘ bodies. This is known as graft versus host disease or GvHD. Researchers at the Technical University of Munich (TUM) and the Universitätsklinikum Regensburg (UKR) have shown that GvHD is much less common when certain microbes are present in the gut. In the future, it may be possible to deliberately bring about this protective composition of the microbiome.