This study is testing how well a drug called abatacept works to prevent Graft Versus Host Disease (GVHD) after a transplant. GVHD is when the new cells from a donor attack the patient's body. The study compares two ways of giving abatacept: a longer treatment versus a shorter one with a placebo (a fake medicine). Patients will get four doses of abatacept and then be randomly chosen to receive either more abatacept or placebo. The main goal is to see if this helps patients avoid severe GVHD and cancer relapse for up to two years after the transplant.

• Study Length: 2 years, with regular follow-ups.
• Visits: Several clinic visits for drug administration and follow-ups.
• Risks: Possible allergic reactions and GVHD.

Patients must be at least 2 years old, weigh 10 kg, and have a donor match. They should not have certain cancers or infections. Everyone must give consent to join. This study helps researchers learn better ways to prevent GVHD in patients receiving transplants.