Hemophagocytic lymphohistiocytosis (HLH)
is a condition in which the body makes too many activated immune cells (macrophages and lymphocytes). People with HLH usually develop symptoms within the first months or years of life which may include fever, enlarged liver or spleen, cytopenia (lower-than-normal number of blood cells), and neurological abnormalities. HLH may be inherited in an autosomal recessive manner or it can have non-genetic causes in which case it is called acquired HLH. There are five subtypes of inherited HLH which are designated familial HLH, types 1-5. Each subtype is caused by a change (mutation) in a different gene. The genetic cause of type 1 is currently unknown. Types 2-5 are caused by mutations in the PRF1 gene, the UNC13D gene, the STX11 gene and the STXBP2 gene, respectively. Treatment depends on a number of factors, including the severity of symptoms, the age of onset, and the underlying cause of the condition. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.