NK cell activity is decreased in HLH, particularly with regard to loss of both cytolytic activity as measured by degranulation, and by IFN-γ production [15,69], the importance of which is highlighted by the fact that patients with influenza and systemic juvenile idiopathic arthritis that developed severe HLH more frequently carry mutations in genes implicated in NK cell cytotoxicity [83,90]. Here, IFNG is linked to hemophagocytic syndrome.