HLH stands for Hemophagocytic Lymphohistiocytosis, which is a rare disease that affects the immune system. In this disease, certain white blood cells, including histiocytes and lymphocytes, attack other blood cells, leading to damage to organs such as the liver and spleen. HLH can occur in both children and adults, and it can be inherited or acquired. Symptoms of HLH include fever, low levels of white blood cells, anemia, and low platelet counts. A diagnosis of HLH is based on symptoms, physical exam findings, and several lab tests. Treatment for HLH may include chemotherapy, blood transfusions, and a gamma-blocking antibody medication called emapalumab.