Histiocytic Disorder Follow-up Study

This is a long-term follow-up registry study for patients diagnosed with histiocytic disorders — a group of rare diseases where certain immune cells (histiocytes) grow abnormally. The registry tracks health outcomes over time. **You may be eligible if...** - You have been diagnosed with a histiocytic disorder at any age, including: - Langerhans cell histiocytosis (LCH) - Erdheim-Chester disease - Rosai-Dorfman disease - Xanthogranuloma - Malignant histiocytosis **You may NOT be eligible if...** - You do not have a confirmed diagnosis of a histiocytic disorder - You are unwilling to participate in long-term follow-up Talk to your doctor to see if this trial is right for you.