HLH is a disorder caused by widespread activation of macrophages and cytotoxic T cells. The macrophages begin to phagocyotse marrow elements which leads to peripheral pancytopenias and their resulting symptoms such as anemia, fever and bruising. It may be idiopathic or secondary to some infections or malignancies. Serum ferritin is quite elevated and bone marrow biopsy shows activated macrophages phagocytosing marrow cells. https://www.grepmed.com/images/11208/lymphohistiocytosis-summary-diagnosis-treatment-management