Safety and Efficacy of Subcutaneous Sarilumab in Improving the Quality of Life in People With Indolent Systemic Mastocytosis

Background: Mast cells help the body fight disease and heal wounds. People with indolent systemic mastocytosis (ISM) make too many mast cells. This causes pain, tiredness, digestive problems, and other symptoms. Researchers think the drug sarilumab could help. Objective: To see if sarilumab is a safe and effective treatment for people with ISM. Eligibility: Adults ages 18-75 with ISM who are enrolled in NIH study 02-I-0277 Design: Participants will be screened with: * Physical exam * Medical history * Blood and urine tests * Questionnaires * Bone marrow removed by a needle inserted into the hip bone * Ultrasound of the abdomen * Photographs of the skin Participants will repeat some screening tests at study visits. Participants will have a baseline visit in the hospital for 3 days. They will: * Be assigned to get either the study drug or a placebo. They will not know which one they get. * Have a skin punch biopsy: An instrument will remove a small piece of skin. * Get their first drug dose injected under their skin Participants will keep a side effect and medication diary during the study. Participants will visit the clinic to get a drug dose every 2 weeks, for a total of 8 doses. Participants will have a visit 2 weeks after their final dose. It will last up to 2 days. Participants will have another visit 12 weeks later. Participants may then continue this study for 1 more year. Those who continue will get sarilumab, even if they previously got the placebo, every 2 weeks. They will have visits every 6 weeks, and then every 3 months.