HLH is a distinct clinical entity characterised by fever, pancytopenia, splenomegaly and haemophagocytosis in bone marrow, spleen, liver or lymph nodes. Laboratory investigations usually reveal high triglyceride and ferritin levels, impaired NK and cytotoxic T-cell function and low fibrinogen. It is a syndrome of macrophage activation, usually secondary to an immunological trigger, resulting in phagocytosis of mature and immature red cells, myeloid elements and platelets. In addition there is intense immune system activation causing release of inflammatory mediators IFNc, TNFa, IL-6, IL-10; Th-1 responses and organ system damage.