Patients with HLH have an abnormally regulated immune system, and specific white blood cells, called macrophages, grow abnormally and accumulate in the body's organs, including the liver, spleen, bone marrow, central nervous system and skin.

There are two main types of HLH: primary and secondary. Primary HLH, also known as familial or relapsing HLH, is an inherited condition. Affected individuals may have an abnormality in a gene that is important in regulation of immune response; some of these gene defects are now known and can be detected. A similar illness, called secondary HLH, may be triggered by certain types of infection, auto-immune diseases and/or by cancer. The treatment of HLH includes chemotherapy. For some patients, bone marrow transplantation is recommended.