Graft-ver­sus-host dise­a­se descri­bes the reac­tion of the trans­plant to the reci­pi­ent. The immu­ne sys­tem of the donor, which is trans­plan­ted with the unre­la­ted dona­ti­on, iden­ti­fies the cells in the body of the reci­pi­ent as for­eign and responds defensively.