Conceptualizing hemophagocytic lymphohistiocytosis (HLH) syndrome. HLH syndrome includes all conditions meeting HLH diagnostic criteria. This syndrome includes conditions of hyperinflammation (“HLH disease,” conditions close to the upper side) that would benefit from HLH-directed immunosuppressive therapies, and those conditions with adequate inflammation (“HLH disease mimics,” conditions close to the lower side) that would not benefit from such therapy. These conditions can be endogenous (those close to the right) or iatrogenic (those close to the left). DRESS, Drug Reaction with Eosinophilia and Systemic Symptoms; MAS, macrophage activation syndrome.