Familial Hemophagocytic lymphohistiocytosis (FHL) is a rare primary immunodeficiency characterized by a macrophage activation syndrome with an onset usually occurring within a few months or less common several years after birth. The disease is Monarch Disease Ontology id MONDO_0009974 (familial hemophagocytic lymphohistiocytosis type 1). Also known as: FHL1, HLH1, HPLH1, familial HLH.