Typical symptoms of HLH besides persistent fever are pallor (paleness), jaundice, liver and spleen enlargement, and neurological symptoms, such as irritability or even seizures. The involvement of the bone marrow, the site of blood cell production, can lead to severe decline of the blood cell counts (red and white blood cells and platelets).  On bone marrow examination, histiocytes that are “eating” other blood cells (also known as phagocytosis) can be detected. Although the disease was named after this phenomenon, it can be absent at the onset or even throughout the course of the disease.

Because symptoms can vary widely, it is sometimes difficult for the physician to make a diagnosis of HLH early in the course of the disease without the help of specialized laboratory tests. To facilitate a rapid and accurate diagnosis, the Histiocyte Society has created diagnostic guidelines and recommendations concerning the treatment of HLH. This is   known as the HLH-2004 treatment protocol.