Hemophagocytic lymphohistiocytosis (HLH) is a rare disease that usually occurs in infants and young children. It may also occur in adults. Children usually inherit the disease. In adults, many different conditions, including infections and cancer, can cause HLH. 

If you have HLH, your body's defense system, called your immune system, does not work normally. Certain white blood cells - histiocytes and lymphocytes - attack your other blood cells. These abnormal blood cells collect in your spleen and liver, causing these organs to enlarge.

Your healthcare provider bases a diagnosis of HLH on your symptoms, physical exam findings, and several lab tests. A prolonged fever is a commonly occurring symptom. An enlarged liver or spleen (located in the upper left side of the abdomen) are key physical findings. Your healthcare provider conducts blood tests to look for:

Low levels of white blood cells called natural killer cells (these cells are important for a healthy immune system)

Low levels of white blood cells, red blood cells, and clotting cells called platelets

High levels of triglycerides (fats in your blood)

Low levels of fibrinogen (a protein important for clotting)

High levels of ferritin (a protein that stores iron)

High levels of a substance called CD25 which increases in your blood when your immune system is stimulated

Another important test, a bone marrow biopsy, calls for taking a sample of your bone marrow (the center of the bone, where blood cells are made) and examining it under a microscope. Other tests may include genetic testing and blood cultures, which are blood samples, to look for infection in your blood.