Hemophagocytic lymphohistiocytosis (HLH) is a condition in which the body makes too many activated immune cells (macrophages and lymphocytes). 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. Each subtype is caused by a change in a different gene. The genetic cause of type 1 is currently unknown. Types 2-5 are caused by genetic changes in the PRF1 gene, the UNC13D gene, the STX11 gene and the STXBP2 gene, respectively. 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 genetic change in the SH2D1A or XIAP gene and is inherited in an X-linked manner.