Graft-versus-host disease (GVHD) is a potentially serious complication that can occur after an allogeneic stem cell or bone marrow transplant, where stem cells are donated from another person rather than from the patient themselves. In GVHD, the donor's immune cells (the graft) recognize the recipient's cells (the host) as foreign and attack them, leading to inflammation and damage in various organs. There are two main types of GVHD:

1. Acute GVHD: Typically develops in the first 100 days after transplant, affecting primarily the skin, gastrointestinal (GI) tract, and sometimes the liver. Symptoms include skin rashes, redness, pain or itchiness, nausea, vomiting, diarrhea, and liver abnormalities.
2. Chronic GVHD: Can develop later and affect many parts of the body including the skin, liver, lungs, and GI tract. Symptoms and severity can vary widely.

GVHD occurs because the donor's immune system cells attack the patient's healthy cells, which is different from transplant rejection, where the recipient's immune system attacks the transplanted organ or tissue. GVHD can range from mild to severe and can sometimes be life-threatening. A mild form of GVHD may actually be beneficial after a transplant for blood cancers because the donor immune cells also help destroy any remaining cancer cells. Treatment involves immune-suppressing medications to reduce the donor cells' attack on the host's body, and managing symptoms based on the organs affected. The risk and severity of GVHD depend on how closely the donor's and recipient's tissues match, with closer matches reducing but not eliminating the risk. In summary, GVHD is a condition where donor immune cells attack the recipient’s body cells after a stem cell or bone marrow transplant, affecting multiple organs and requiring careful management. It is a key consideration in the success of allogeneic transplants. If you want, I can provide more detailed information on symptoms, treatment, or prevention strategies for GVHD.