The main defect in MAS is deficient NK cell activity. Hence a trigger which stimulates the immune system leads to unchecked CD8+ T cell proliferation which persists even after elimination of the trigger. These T cells secrete IFN-gamma which leads to macrophage overstimulation. This in turn activates a cytokine storm production (TNF a, IL 1 and IL 6) and phagocytosis of marrow elements by the marcophages, ultimately leading to MODS7. MAS usually occurs in the setting of an untreated or inadequately controlled rheumatic disease with severe systemic inflammation. The diagnosis is difficult particularly against this background and therefore a high clinical suspicion is paramount.