Hemophagocytic Syndromes

Hemophagocytic lymphohistiocytosis (HLH) is a rare disorder of inflammation that was first thought to affect only young infants and children but is increasingly recognized in older children and adults.

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.

Latest HLH Educational Webinar

Play Video


The symptoms of the primary and secondary forms of hemophagocytic lymphohistiocytosis (HLH) include:

  • Persistent fever, often high. Sometimes a microbial (often viral) cause is found, but this is common as a trigger of HLH/FHL.
  • Abnormal blood tests such as a) low red blood cells, b) low platelets, and c) low neutrophils.
  • Other blood abnormalities (liver dysfunction, elevated triglycerides, low sodium level, and low albumin level).
  • Enlarged spleen.
  • Skin rash, variable (30% of patients).
  • Enlarged lymph nodes.

Combined with evidence of immunologic dysfunction:

  • Decreased NK-cell function.
  • Increased T-cell activation as evidenced by increased sIL2ra (soluble IL-2 receptor alpha) in the blood.
  • Increased histiocyte activation as evidenced by increased ferritin.

CNS (central nervous system) involvement occurs in 50% of patients and may be present at the time of disease onset, or it can develop later.

A wide variety of symptoms of CNS disease include:

  • Irritability.
  • Seizures.
  • Fatigue.
  • Low or abnormally increased muscle tone.
  • Difficulty with coordination.
  • Weakness of face/eye nerves.
  • Sudden blindness.
  • Paralysis and coma (very rare).

On bone marrow examination, an abnormality in which histiocytes are “eating” other blood cells (also known as hemophagocytosis) can be detected. When prominent, this is a very useful diagnostic marker. Although the disease was named after this phenomenon, this symptom may be absent at onset or even throughout the course of the disease.

The Histiocyte Society has created diagnostic guidelines and recommendations concerning the treatment of HLH.  The HLH-2004 research study, concluded at the end of 2011, is used only at institutions with IRB (Institutional Review Board) approval and appropriate informal patient/family consent.   The Histiocyte Society recommends the use of the HLH-94 treatment in conjunction with HLH-2004 diagnostic criteria as the standard of care until further analysis/publications recommend otherwise.

Diagnosis and Treatment

In January 2023, the Histiocyte Society published HLH/MAS Consensus Management Efforts, Expert Help Diagnosing and Managing HLH, which curates the most recent consensus-based guidance manuscripts, organized by the HS and/or affiliated organizations and authored by HS experts, to assist with the diagnosis and management of HLH across many different contexts. Read more here.

It is sometimes difficult to establish the diagnosis of hemophagocytic lymphohistiocytosis (HLH), and the combination of the physical symptoms and certain laboratory tests is required.   A set of diagnostic criteria was recommended by the Histiocyte Society for use in the HLH-2004 research protocol, and this was revised in 2007.  This includes diagnosis of a specific gene defect and/or the presence of at least five of the following eight criteria:  (Note:  The understanding of pathology underlying HLH/FHL disease is evolving, and recommended “diagnostic” criteria will be revised in upcoming years.)

  • Low or absent NK (natural killer) cell function.
  • Prolonged fever.
  • Blood cell abnormalities (low white cells, low red cells, low platelets).
  • Enlarged spleen.
  • Increased triglycerides (fat) or decreased fibrinogen (protein necessary for clotting) in the blood.
  • Increased ferritin (protein that stores iron) in the blood.
  • Abnormal bone marrow test evidence of hemophagocytosis but not malignancy or other cause.
  • Abnormally high CD25 (also known as sIL2ra) in the blood indicating abnormally increased T-cell activation.

The test for low or absent natural killer cell (NK) function has been found useful in assisting to make a clinical diagnosis of HLH.  This abnormality is found in many  patients with FHL, as well as in many cases of secondary disease but rarely in the X-linked forms.

However, it is just one piece of information and should not be used to determine the diagnosis of HLH as primary or secondary.  NK function cannot be determined before birth, and it may not be reliably studied until a child is at least 6 weeks of age. FHL is suspected if siblings have been diagnosed with HLH, if symptoms intensify during treatment for HLH, or if symptoms return after therapy has been stopped.

Since it is difficult to tell the difference between secondary HLH and FHL, any case of HLH should be considered for genetic testing to confirm the diagnosis.  Since 1999, at least seven defective genes have been identified.  Autosomal recessive:  PRF1 (perforin), MUNC13-4, STX11 (Syntaxin), STXBP2, and RAB27A.  X-linked:  SH2D1A, BIRC4.

There are some FHL patients (approximately 30%) with no identified gene defect, so normal genetic test results do not necessarily rule out the diagnosis of FHL.  Genetic testing is usually done on blood, although other kinds of tissue samples can be used.  Once the genetic cause is known, the parents can quickly be tested to confirm that they are carriers for that specific genetic type of FHL. Other siblings can also be easily tested, even before birth, once the genetic cause of the disorder in the family is known.  Even in the event of death, salvaged tissue can be tested to determine if siblings are at risk.

It is recommended that genetic testing be coordinated through and requested by a referring physician or genetic counselor.  Test results are generally available in about one month to six weeks. The cost of genetic testing is covered by most insurance companies.  There are several hospitals that provide information and perform genetic testing on a clinical basis, including:

Cincinnati Children’s Hospital, Cincinnati, Ohio

Karolinska University Hospital, Stockholm, Sweden
Jan-Inge Henter, MD

University Medical Center, Hamburg, Germany
Gritta Janka, MD

Gene Tests Clinic Directory

New Treatment Approved in 2018

US Federal Drug Administration (FDA) Approves Gamifant® (emapalumab-lzsg), the First and Only Treatment Indicated for Primary Hemophagocytic Lymphohistiocytosis (HLH). Read the full press release to get more information here.
*This is not an endorsement of any specific treatment or drug. All treatment should be reviewed by your medical team to ensure the best treatment plan is developed for you.


Participation in research can help us all better understand the natural history of HLH, the role of genetics, permanent effects, as well as which treatments are most effective.

The INTO-HLH Registry: (Insight into the Natural history and Treatment Outcomes of Hemophagocytic Lymphohistiocytosis) will enroll pediatric and adult patients with HLH. The proposed study, a collaboration between Cincinnati Children's Hospital Medical Center (CCHMC), Texas Children's Hospital, the North American Consortium for Histiocytosis (NACHO), and Sobi Inc. aims to establish a robust registry that will enable investigators to better define the natural history of HLH. FAQs for the study can be found here, and the clinicaltrials.gov page, here. Visit the study website: https://hlhregistry.org. Download the Brochure or Scan the QR Code

Adult HLH Registry - Europe: A register database was established at the Jena University Hospital for the systematic collection of patient cases with HLH in adults. This web-based register database was created in cooperation between the Jena Center for Clinical Studies and the Department of Hematology and Oncology at the Jena University Hospital. Clinical and laboratory chemical data are registered in an anonymous form.

Histiocyte Society (HS) and European Society of Immunodeficiencies (ESID) HLH Registry: The HLH Registry is a database implemented as a collaborative project of the HS and ESID in collaboration with the Inborn Errors Working Party (IEWP) of the European Bone Marrow Transplantation Group (EBMT) but is not limited to formal members as participants worldwide are encouraged to contribute to this effort. This international HLH Registry is aimed to collect and analyze clinical, epidemiological, immunological, genetic, treatment and survival data for assessment of feasibility and design of future interventional studies on the treatment of HLH.

*If you have already enrolled in one of the above studies, you are welcome to enroll in the others, so long as you meet the eligibility criteria. You can also ask your physician or the registry's team if enrolling in one automatically enrolls you in another. Multiple registries exist due to the need to focus on a particular set of information, such as geographic location (i.e., Europe v North America). The registry information or team will inform you about collaborative efforts with other existing registries or studies.*

Additional Information

In 1994, as a result of an international cooperative effort, the Histiocyte Society created the first treatment protocol for patients with HLH/FHL.  This included a combination of chemotherapy, immunotherapy and steroids, as well as antibiotics and antiviral drugs, followed by a stem-cell transplant in patients with persistent or recurring HLH or those with FHL.  The HLH-2004 protocol was based on the HLH-94 protocol with minor changes such as cyclosporin, an immunosuppressant drug, being started at the onset of therapy rather than week #8.  This protocol has been widely accepted internationally and is used in numerous countries on all continents but should only be used as a research study.  HLH-2004 closed at the end of 2011. A new pilot clinical trial for the treatment of HLH, called "HIT HLH," has opened in the U.S. at a network of participating sites (see www.clinicaltrials.gov) and a companion study is expected to open in Europe soon. This protocol combines two effective strategies for HLH treatment. In order to participate in this trial, patients must be treated at a participating center. There is also an update article available about the study and the role played by interferon gamma (INFγ) in the development and maintenance of HLH available here.

There are cases of secondary HLH that can resolve spontaneously or after treatment of the underlying disease, without the use of chemotherapy.  The treatment should be guided in part by the severity of the condition, as well as the cause of the disease.

FHL, however, when not treated, is usually rapidly fatal with an average historical survival of about 2 months.  The treatment included in the HLH-2004 research protocol is intended to achieve stability of the disease symptoms so that a patient can then receive a stem-cell transplant, which is necessary for a cure.

In recent years, some transplant centers have adopted the use of reduced intensity conditioning (or “RIC”) to prepare for the stem cell transplant. This approach offers the possibility of better survival with stem cell transplant than the intensive chemotherapy protocols previously used.

As research continues, the outcome for patients with HLH/FHL has improved greatly in recent years.  Approximately two-thirds of children with HLH who undergo transplantation can expect to be cured of their disease.  However, there are a number of complications that can occur during the process of transplant, including severe inflammatory reactions, anemia, and graft-versus-host disease.

Long-term follow-up of survivors of transplants for HLH/FHL indicates that most children return to a normal or near-normal quality of life.  The results of transplantation are generally better when the procedure is performed at a major pediatric transplant center where the doctors are familiar with this disease.  Early and accurate diagnosis is essential.  However, there is still a high rate of death, indicating the education of the medical community regarding prompt diagnosis and management of the diseases, as well as improvement in managing therapy to provide optimal outcome still requires support for clinical research of this group of disorders.

Locate a physician:


What Do I Do Now?

A new diagnosis of a histiocytic disorder can bring feelings of being overwhelmed, bewildered, scared, angry, sad, disappointed, helpless, and sometimes even feelings of guilt.  It is a time filled with unknowns, change, and new people and situations.  Most parents feel ill-equipped to understand what is happening to their child and how to navigate through the illness to recovery.  There are, however, strategies and resources that can help you, your child, and other family members get through the uncertain times that lie ahead. One step at a time, you can gather information, create a support system, learn how to cope with stress, and become a strong advocate for your Histio Warrior as part of the medical team. The following tips and suggestions are provided to help guide you through this journey toward your Histio Warrior's good health.

Printable Fact Sheets

Learn more about the different types of histiocytic disorders from these helpful fact sheets. Printing these for family and friends is a quick and easy way to teach them about the disease. More extensive information can be found in the Disease Information section of our website.

Choose from the following Fact Sheets:

Histiocytic Disorders Overall FAQ