HLH stands for hemophagocytic lymphohistiocytosis.  HLH is a life-threatening condition.  It is often caused by an inherited problem of the immune system, which is called “primary” HLH or “familial” HLH. 

In patients with primary HLH, cells of the immune system, principally T cells and NK cells, don’t work properly to destroy infected or damaged cells as they should.  Because of this, the immune system becomes overstimulated and over activated.  The immune system then begins to damage the patient’s own tissues and organs, including the bone marrow, the liver and the brain. 

Doctors can even sometimes see cells of the immune system “eating” other cells when they look at the bone marrow from a patient.  This is called hemophagocytosis.  

Some patients are diagnosed with what is called “secondary” HLH.  This term is used when your doctor thinks your HLH may have occurred for a variety of different reasons, but not necessarily because of an inherited condition associated with abnormal function of the immune system.