GVHD occurs when the donor T cells in the transplanted stem cells attack the recipient's tissues. The severity of GVHD can vary from mild to life-threatening, and it primarily affects the skin, liver, and gastrointestinal tract. There are two types of GVHD: acute and chronic. Acute GVHD typically develops within the first 100 days post-transplant, while chronic GVHD can occur at any time thereafter.