Hemophagocytic Lymphohistiocytosis Abstract. Hemophagocytic lymphohistiocytosis (HLH) is a group of rare diseases characterized by over-activation of the immune system. They form two groups: primary and secondary HLH. Primary HLH are linked to mutations impairing lymphocyte cytotoxicity. Secondary HLH are triggered by infections, autoimmune diseases or neoplasia, the remaining cases being labeled idiopathic. HLH manifest as febrile states, cytopenias and hepatosplenomegaly. In the absence of treatment, they quickly lead to multiple organ failure. The diagnosis is currently based on the presence of several clinical and biological markers. Treatment consists of suppression of the triggering factor, organ support and immunosuppression. Primary forms, affecting a pediatric population, have been the subject of intense research, and are nowadays treated with established therapeutic protocols. Several recent retrospective studies have improved our knowledge of secondary HLH, which affects mostly adults and whose incidence seems to be increasing. Thus, new diagnostic criteria are currently being studied for secondary HLH, and several treatment protocols have just been published or are being evaluated.