 This is an investigational study. Alemtuzumab, etoposide, tocilizumab, and dexamethasone are not FDA approved for the treatment of HLH. Etoposide is FDA approved and commercially available for the treatment of testicular cancer and lung cancer. Alemtuzumab is FDA approved and commercially available for the treatment of chronic lymphocytic leukemia. Dexamethasone is a steroid used to reduce inflammation. Tocilizumab is FDA approved and commercially available for the treatment of arthritis. The combination of alemtuzumab, etoposide, tocilizumab, and dexamethasone to treat HLH is investigational. The study doctor can explain how the drugs are designed to work. 