According to a large, population-based study from Sweden, it was estimated to occur in 1.2 cases per million children, corresponding to 1 in 50,000 births. However, this number is probably an underestimation as physicians appear to recognize HLH more readily now than when this was published decades ago. According to a large Japanese study, around 40% of cases occur in adults.
HLH occurs when the immune system is triggered (most often by a virus) and overreacts in specific ways that make the patient severely ill. Thus, HLH is best thought of as a problem of proper immune regulation. Poor immune regulation may be caused by genetic defects (called familial HLH, or FHL) or it may occur in patients with various infections, rheumatologic diseases, or cancers, for reasons that are not entirely understood (sometimes called secondary HLH). The HLH process is like a storm of inflammatory factors, called cytokines, which involves many organs and various immune cells (macrophages/histiocytes and lymphocytes, the cells referred to in HLH’s name).
In the familial form of HLH, FHL, defective genes are inherited from both parents (called ‘autosomal recessive’) or more rarely, inherited from just the mother (called ‘X-linked’). While great progress has been made through research in recent years to define these genes, there remains a portion of FHL patients with yet unknown underlying gene defects. Most patients with FHL get sick before the age of 1 year. Though two X-linked forms usually only affect males, other genetic causes are seen equally in males and females. Because FHL runs in families and tends to reoccur in an affected child (if not treated), other family members including siblings may also be affected. Each full sibling of a child with the most common forms of FHL has a 25% chance of developing the disease, a 50% chance of carrying the defective gene (which usually does not give any risk of disease), and a 25% chance of not being affected and also not carrying the gene defect.
Secondary/reactive HLH is usually diagnosed in older patients with no family history of this disease. When HLH occurs in someone who has a rheumatologic disorder (most commonly systemic onset juvenile idiopathic arthritis, or so-JIA) or cancer, the HLH is best viewed as a complication of these conditions, or ‘secondary’ to these underlying problems. HLH also occurs in children and adults after infections, particularly viral infections such as Epstein-Barr virus (EBV) and cytomegalovirus (CMV). Similarly, HLH can be thought of as secondary to these infections, though infection also triggers FHL, so the presence of infection does not define the underlying cause of HLH. Finally, immune-activating treatments for cancer such as CAR-T therapy may cause a cytokine storm, which shares many features with HLH.
If someone has familial HLH, also referred to as primary HLH, the HLH will likely recur once treatment is stopped. To prevent it from returning, most of these patient receive a bone marrow transplant from someone who is not at risk for HLH, either a family member or an unrelated volunteer donor. The new bone marrow replaces the recipients immune system with a healthy one. Learn more about bone marrow transplant.
It is difficult to know whether a patient has familial or secondary HLH based on symptoms, which may be very similar. Therefore, genetic testing is usually recommended for young children or adults with recurrent or unexplained HLH. Learn more about genomic profiling (genetic testing).
The first description of HLH was published in 1952, but it has only been in recent years that it has received more widespread attention. In 1985, physicians worldwide interested in studying histiocytic disorders including HLH gathered in Philadelphia and formed the Histiocyte Society. Funds raised by the Histiocytosis Association and national subgroups have financed research that has led to significant breakthroughs in the diagnosis and treatment of HLH during the last decades. As awareness and understanding of this disease have increased worldwide, the diagnosis and survival rates have improved significantly. However, HLH remains a rapidly progressive disease requiring effective anti-inflammatory therapy.
The information on this page has been written and reviewed by the Histiocytosis Association Board of Trustees Scientific Committee and a member of the Histiocyte Society, and subsequently audited by patients and families to ensure enough information was captured. The most recent update to this page was in August of 2023.
