This study is for patients with **Sickle Cell Disease (SCD)**, a condition where red blood cells are misshaped and can cause blockages in blood flow. The trial will test if a medicine called **daratumumab** can be used before a special type of transplant to help patients who have antibodies (proteins made by the immune system) that could attack donor red blood cells. This approach uses a "nonmyeloablative" method, meaning the treatment is less intense on the body compared to traditional transplants. The goal is to see if this treatment can prevent a condition called **pure red blood cell aplasia**, where red blood cells are not made properly, and help patients live without complications similar to those without these antibodies. **Key Points:** - **Eligibility:** Ages 2-24.99 with a matched sibling donor and certain health conditions. - **Study Duration:** A 4-week period before the transplant where daratumumab is given. - **Risks:** Patients must not have serious infections, heart, liver, or kidney problems. This study hopes to offer a safer transplant option for patients with SCD, providing important information on using daratumumab before transplants.