CD80 and graft versus host disease: The MSCs’ ability to limit self-attacking responses is consistent with their low level of co-stimulatory factors CD-80 and CD-86 (MHC I), which are essential for the MHC class II activation and for the triggering of T cells’ second immune response; this is similar to what happens during a severe graft-versus-host disease (GVHD) and during the last phase of COVID-19 infection.