This research is studying a medicine called **axatilimab** for people with a condition called chronic graft versus host disease (**cGVHD**). This is a condition where the immune cells from a bone marrow or stem cell transplant attack the body of the person who got the transplant. The study will look at how well axatilimab works, if it's safe, and how people feel taking it. **Key Points:** - The study lasts up to **2 years** with treatments every **28 days**. - Participants must have tried at least **2 other treatments** that didn't work well. - The study includes multiple centers, and participants will receive 1 of 3 axatilimab doses. To join, you must be over 2 years old, have active cGVHD, and be in good health otherwise. You can't join if you have acute GVHD without cGVHD, certain infections, or if you're pregnant. You must not have other serious health issues or be in another study. Participants may use certain medications like corticosteroids, but not others unless approved.