This study is about a medicine called **ravulizumab** for people who had a special type of blood cell transplant, called a hematopoietic stem cell transplant (HSCT), and developed a problem called thrombotic microangiopathy (TMA), which is a condition where small blood vessels get blocked. The study has two parts: the first part is open-label, meaning everyone knows what treatment they're getting, and the second part is a blind test where participants won't know if they're getting ravulizumab or a placebo (a "dummy" treatment). The treatment lasts for 26 weeks, followed by another 26 weeks of observation. To join, you need to be at least 12 years old, weigh at least 30 kilograms, and have had an HSCT in the last year. There are also other rules about health and vaccinations. If you have certain health issues, you might not be able to join.

• The study lasts for 52 weeks total, with 26 weeks of treatment.
• Participants must have had an HSCT within the past year.
• People with certain health conditions or histories cannot participate.