**Study Summary** This study aims to understand why some people with **Sickle Cell Disease (SCD)** have an immune reaction when they receive a blood transfusion. A blood transfusion is when you receive blood from another person. The immune reaction is called an *alloimmune response*. Researchers believe that this response is linked to changes in our genes, which are tiny instructions in our cells that tell our body how to work. By comparing genes of those who have an alloimmune response to those who don't, scientists hope to find ways to improve treatments. **Key Points:** - **Length & Visits:** Participation involves gene analysis; the details on visits or length aren't specified. - **Eligibility:** Participants must have SCD, be over 2 years old, and meet specific transfusion history or genetic criteria. - **Exclusions:** People with impaired decision-making, transplants, on certain medications, or pregnant cannot join. This research could lead to better care for people with SCD who have reactions to blood transfusions.