This study aims to find out how common a specific gene change, called **KIT D816V mutation**, is in people with certain types of mast cell disease. Mast cells are a part of your immune system that help with allergic reactions. When these cells don't work right, it might cause problems like **anaphylaxis** (a severe allergic reaction), **mastocytosis** (too many mast cells), or other symptoms without a clear cause. Participants are divided into four groups based on their health history and symptoms. The study is held at multiple locations, and doctors will use your medical history and some tests to see if you have the mutation.

• **Length of Study:** You may need to visit multiple times for tests and follow-ups.
• **Eligibility:** You must fit one of four specific health categories to participate.
• **Risks:** No known major risks, but some tests might cause minor discomfort.

This study is not for those who already know they have certain mast cell-related conditions or specific gene changes. Participation might include tests like blood draws or other medical exams.