Graft-versus-host disease (GVHD) occurs when transplanted immune cells from a donor attack the recipient's body. This often happens after procedures like a bone marrow transplant, where the donor's immune cells recognize the recipient's tissues as foreign. Symptoms can include skin rashes, liver problems, and gastrointestinal issues. GVHD can be acute or chronic. Acute GVHD typically appears within the first 100 days post-transplant, while chronic GVHD may develop later and can last for months or years. Treatment often involves medications to suppress the immune response, helping to manage symptoms and improve the recipient's quality of life.