HFS is an activation of mononuclear phagocyte system cells, with hemophagocytosis in bone marrow and the rest of reticuloendothelial system. This syndrome can be either primary/familial (familial hemophagocytic lymphohistiocytosis -FHL) or reactive/secondary. FHL has a recessive autosomal inheritance and it develops in chil- Bone marrow smear (optic microscope) Figure 1 Bone marrow smear (optic microscope). Activated macrophages and hemophagocytosis from bone marrow puncture-biopsy.