This study is looking at how well a medicine called *itolizumab* works when used with *corticosteroids* to treat a condition known as *acute Graft Versus Host Disease (aGVHD)*. aGVHD happens when cells from a donor attack the body of a person who got a stem cell transplant. This study will compare itolizumab with a fake treatment (placebo) to see which works better. The study will have 200 participants from all over the world. People in the study will get the medicine through a needle in their vein 7 times every two weeks, and doctors will check on them for about a year. To join, you should be 12 years or older, weigh more than 40kg, and have started steroid treatment for aGVHD within the last 72 hours.

• The study lasts around a year with 7 treatments given every two weeks.
• Participants must be 12 years or older and weigh over 40kg.
• No other major illness should be present, except for certain skin or breast cancers.