What Is HLH?

HLH stands for Hemophagocytic Lymphohistiocytosis. HLH is a rarely diagnosed life-threatening condition. HLH qualifies as an orphan or rare disease since there is currently little government funding for research. 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, do not 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 HLH has occurred for a variety of different reasons, but not necessarily because of an inherited condition associated with abnormal function of the immune system. (Monique’s form of HLH was secondary brought about by the EBV (mono) virus).

Ultimately, children and young adults who have HLH on the basis of known genetic defects will require a hematopoietic cells transplant (BMT) to cure them. Patients without a known genetic cause who experience progressive HLH or recurrent episodes are also considered for BMT.

The Doctors at Cincinnati Children’s Hospital treat more cases of HLH each year than any other pediatric hospital in the world.