A rare blood disorder, seen more often in children than adults, in which the body produces too many immune cells called lymphocytes and macrophages, which then attack the body’s own tissues and organs, including the brain, liver, and bone marrow.

HLH can be inherited (called primary or familial) or acquired (called secondary). Symptoms – usually beginning during the first months of life – may include an enlargement of the liver or spleen, fever, a decreased number of blood cells, skin rash, breathing and heart problems and neurologic abnormalities.

An allogeneic hematopoietic cell transplant is considered a cure for familial HLH.