Subjects with hemophagocytic lymphohistiocytosis (HLH) often have life-threatening complications at the time of diagnosis resulting from excessive inflammation. This excessive inflammation is driven by abnormally high levels of cytokines--proteins involved in inflammation. Standard therapy for HLH does not directly target these cytokines. Tocilizumab is a medicine that blocks one of the cytokines that is elevated in patients with HLH.