This study tests a new treatment for a condition called acute graft versus host disease (aGVHD). In aGVHD, the body's immune system attacks its own cells after a bone marrow transplant. The study compares a drug called itolizumab to a placebo (a "fake" treatment) to see if it helps patients with severe aGVHD. All patients will also take corticosteroids, a type of medicine that reduces swelling and immune responses. **Participants** will be randomly assigned to receive either itolizumab or the placebo. The treatment includes seven doses given through a vein, about every two weeks. The study lasts for about one year.

• Participants must be 12 years or older and weigh more than 40 kg (about 88 pounds).
• They must have started taking corticosteroids for aGVHD within 72 hours before starting the study medication.
• The study involves multiple visits and follow-ups over the course of one year.