Provided herein is a method and kit for diagnosing and treating Graft versus host disease (GVHD), the method including obtaining a body fluid or tissue sample from said mammal, throughout the year post-transplant; detecting the expression level, phosphorylation level, or a combination thereof, of biomarkers in said body fluid or tissue sample; diagnosing said mammal with Graft versus host disease (GVHD) when the expression level, phosphorylation level, or a combination thereof of said biomarkers in said body fluid or tissue sample are detected with a deviation of at least about 25% from standard measurements taken from healthy subjects and/or from measurements taken from said mammal prior to said transplant; and, administering an effective amount of at least one immunosuppressant or steroid or a combination thereof to the diagnosed mammal.