HLH stands for hemophagocytic lymphohistiocytosis. It is a life-threatening condition that can be defined as either primary or secondary HLH. Primary, or “familial,” HLH is when the condition is inherited. The term secondary HLH is used when your doctor thinks the condition may have occurred for a variety of other non-inherited reasons.

Patients with primary HLH have cells of the immune system called T and NK cells that don’t work properly. These cells become overactive, causing too much inflammation. Ordinarily, these cells should destroy infected, damaged cells of the body. In HLH, the immune system begins to damage the patient’s own tissues and organs, including the liver, brain and bone marrow where blood is made.

Sometimes, doctors can see these angry immune cells “eating” other cells when they look at the bone marrow from a patient using a microscope. This process is called hemophagocytosis.