Medicine
Diagnostic Criteria and Treatment of Hemophagocytic Lymphohistiocytosis
Quick fact
In HLH, serum ferritin levels can exceed 10,000 ng/mL, a striking elevation that is a diagnostic hallmark and often contributes to the early recognition of this rapidly fatal syndrome.
Why this is interesting
You probably know that fever and enlarged organs can mean an infection—but what if it's your own immune system attacking your body? And how can a simple blood test for iron levels save a life?
Read the full explanation
Understanding Diagnostic Criteria and Treatment of Hemophagocytic Lymphohistiocytosis
Imagine your immune system as a fire alarm. Normally, it detects an intruder, sounds the alarm, and then calms down once the threat is gone. In HLH, the alarm gets stuck on, and the fire trucks (immune cells) keep pouring in, releasing chemicals that cause inflammation everywhere. This leads to high fever, an enlarged spleen and liver, and low blood counts because the activated cells consume them. To diagnose HLH, doctors use a checklist called the HLH-2004 criteria. If a patient meets at least 5 of 8 criteria—including fever, splenomegaly, low blood counts, high triglycerides or low fibrinogen, high ferritin, hemophagocytosis (cells eating other cells) in bone marrow or other tissues, low NK cell activity, and high soluble CD25—then HLH is suspected. These criteria help separate this rare hyperinflammatory condition from more common infections or cancers.
A deeper explanation
Why does HLH happen? The underlying mechanism is a failure of the immune system's 'off switch.' Normally, cytotoxic T cells and natural killer (NK) cells kill infected cells and also regulate the immune response by eliminating overactivated antigen-presenting cells. In HLH, mutations in genes like PRF1 (perforin) or UNC13D impair this killing, so the immune response continues unchecked. The resulting cytokine storm—especially high levels of interferon-gamma and tumor necrosis factor—drives macrophages to proliferate and hemophagocytose (ingest blood cells), causing cytopenias and organ damage. Treatment targets this hyperinflammation. For moderate cases, corticosteroids like dexamethasone and immunosuppressants like cyclosporine A are used. For severe or familial cases, etoposide, which inhibits topoisomerase II and induces apoptosis in activated T cells, is added. The ultimate cure for genetic forms is allogeneic hematopoietic stem cell transplantation, which replaces the defective immune system. Even in secondary HLH (triggered by infection or malignancy), controlling the trigger is crucial. Understanding this cascade explains why prompt recognition and combined immunosuppression are life-saving.