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. Symptoms may include fever, enlarged liver or spleen, cytopenia (decreased 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 as 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.

When HLH results from an inappropriate immune response to Epstein-Barr virus or another viral illness, it may be due to a separate genetic condition called X-linked lymphoproliferative disease (XLP). XLP is caused by a mutation in the SH2D1A or XIAP gene and is inherited in an X-linked manner.