Hemophagocytic lymphohistiocytosis (HLH) is a rare, life-threatening disorder that occurs when the immune system becomes overactive and begins to attack the body's own cells and tissues. It is characterized by the accumulation of immune cells (histiocytes and lymphocytes) in various organs, including the liver, spleen, and bone marrow, which can lead to organ damage and dysfunction.

HLH can be inherited (primary HLH) or acquired (secondary HLH). Primary HLH is caused by genetic mutations that affect the immune system, and it is typically diagnosed in infants or young children. Secondary HLH is triggered by an underlying medical condition, such as an infection, autoimmune disorder, or cancer, and it can occur at any age.

Symptoms of HLH may include fever, fatigue, rash, weight loss, organ failure, and abnormal bleeding. The condition can be difficult to diagnose because the symptoms are similar to those of other diseases.

Treatment for HLH typically involves a combination of medications to suppress the immune system and remove excess immune cells, as well as supportive care to manage symptoms and prevent complications. In severe cases, bone marrow transplantation may be necessary.