This study looks for a specific gene change, *KIT D816V*, in people who might have a type of immune system problem called "mast cell disease." Mast cells are cells that help protect our bodies, but sometimes they can cause problems. Participants will be grouped based on their symptoms, like unexplained allergic reactions or certain bone or blood conditions. **Key Points:** - The study will happen in different locations and needs participants with specific symptoms or conditions. - Participants must not have certain known mast cell problems or other specified conditions. - This study is to screen and understand who might have the *KIT D816V* gene change, not to provide treatment. If you have unexplained allergic reactions or certain health conditions, this study might be important for understanding your health. Always talk to a doctor to see if joining is right for you. **Note**: This is just a screening study, so there won't be any treatment provided.