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PubMed This is a summary of 65 peer-reviewed journal articles Updated
Pediatric Hematology

Facing a New Diagnosis of FHL

At a Glance

Familial Hemophagocytic Lymphohistiocytosis (FHL) is a rare genetic disorder where a child's immune system cannot turn off, causing severe inflammation. The immediate treatment goal is calming this 'cytokine storm' with medication, followed by a stem cell transplant as the only known cure.

Hearing that your child has Familial Hemophagocytic Lymphohistiocytosis (FHL) is a moment of profound shock. It is a rare, life-threatening, and hyperinflammatory disorder that requires immediate medical action [1][2]. You are likely feeling overwhelmed, but understanding the nature of this condition can help you find your footing in the days ahead.

Because FHL is so rare—with an estimated incidence of about 0.12 per 100,000 live births—it is quite possible that your local hospital or pediatrician has never encountered it before [3]. This makes it essential to work with specialized pediatric hematologist-oncologists who have experience with this specific immune dysregulation [4][5].

Three Stabilizing Facts

In the midst of this crisis, these three pillars of information can help you understand the path forward:

  1. The “Genetic Switch” is Stuck: FHL is caused by inherited genetic mutations (such as in the PRF1 or UNC13D genes) that damage the “off switch” of the immune system [2][6]. Specifically, the cells that should kill off infected or damaged cells (cytotoxic T lymphocytes and natural killer cells) cannot function correctly [2][7]. This causes the immune system to remain in a state of unbridled activation, leading to a “cytokine storm”—a massive overproduction of inflammatory chemicals that can damage the body’s own organs [8][2].
  2. The Immediate Goal: Calm the Storm: The first priority of treatment is to achieve remission, which means bringing the hyperinflammation under control [1][9]. Doctors use specialized protocols involving chemotherapy and immune-suppressing drugs to “calm the storm” and stabilize your child’s condition [10][1]. This phase is a critical bridge to the next step.
  3. The Ultimate Goal: A New Immune System: Because FHL is a primary genetic condition, medications alone cannot cure it. The only known curative treatment is an allogeneic hematopoietic stem cell transplant (HSCT) [11][12]. In this procedure, your child’s faulty immune system is replaced with healthy donor stem cells that have a working “off switch” [1][12].

Navigating the Guide

This guide is designed to help you understand your child’s diagnosis, the treatment roadmap, and what life looks like after treatment.

01

The Biology and Diagnosis of FHL

Learn about the biology and diagnosis of Familial Hemophagocytic Lymphohistiocytosis (FHL). Understand the HLH-2004 criteria, genetic causes, and symptoms.

02

The Path to a Cure: Treatment and Transplant

Learn about the two-phase treatment for Familial Hemophagocytic Lymphohistiocytosis (FHL). Understand the HLH-94 protocol, emapalumab, and stem cell transplants.

03

Building Your Care Team and Protecting Your Family

Learn how to build an expert medical team for Familial Hemophagocytic Lymphohistiocytosis (FHL). Understand essential specialists and sibling genetic screening.

04

Survivorship and the Road Ahead

Learn about life after a stem cell transplant for familial hemophagocytic lymphohistiocytosis (FHL). Understand late effects, monitoring, and mental health.

Moving Toward Specialized Care

The clinical complexity of FHL requires a high level of coordination [13][14]. Organizations like the Histiocytosis Association and the Histiocyte Society provide crucial resources for families and help guide doctors through established treatment protocols [15][16].

Early genetic testing is standard for young children with these symptoms to confirm the diagnosis and guide the medical team [17][18]. While this is a difficult journey, modern transplantation techniques have significantly improved survival outcomes for children with FHL [12][19].

Common questions in this guide

What causes FHL in children?
FHL is caused by inherited genetic mutations that damage the immune system's 'off switch'. This prevents specialized immune cells from functioning correctly, leading to a massive overreaction and the dangerous overproduction of inflammatory chemicals called a cytokine storm.
Is there a cure for Familial Hemophagocytic Lymphohistiocytosis?
Yes, the only known curative treatment for FHL is an allogeneic hematopoietic stem cell transplant (HSCT). While chemotherapy and immune-suppressing drugs are used initially to calm the severe inflammation, a transplant is needed to replace the child's faulty immune system with healthy donor cells.
What is the first step in treating FHL?
The immediate priority is to achieve remission by bringing the severe inflammation under control. Doctors use specialized protocols involving chemotherapy and immune-suppressing drugs to stabilize your child's condition so they can safely proceed to a stem cell transplant.
Do siblings need to be tested if my child is diagnosed with FHL?
Because FHL is an inherited genetic condition, your medical team will likely recommend genetic testing for siblings. This helps identify the specific mutation, informs future family planning, and determines if siblings might be affected or could serve as potential stem cell donors.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Has our medical team treated children with FHL specifically, rather than just secondary HLH?
  2. 2.Are you currently in contact with a specialist center or the Histiocyte Society regarding my child’s case?
  3. 3.What specific genetic mutation was identified, and what does that mean for our other children or future family planning?
  4. 4.How will we know when the 'cytokine storm' has been sufficiently calmed to move toward transplant?
  5. 5.What is the timeline and process for finding a stem cell donor for my child?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (19)
  1. 1

    Clinical, Genetic, and Outcome Characteristics of Pediatric Patients with Primary Hemophagocytic Lymphohistiocytosis.

    Nepesov S, Yaman Y, Elli M, et al.

    Turkish archives of pediatrics 2022; (57(4)):398-405 doi:10.5152/TurkArchPediatr.2022.21314.

    PMID: 35822471
  2. 2

    Clinical, immunological and genetic findings in patients with UNC13D deficiency (FHL3): A systematic review.

    Amirifar P, Ranjouri MR, Abolhassani H, et al.

    Pediatric allergy and immunology : official publication of the European Society of Pediatric Allergy and Immunology 2021; (32(1)):186-197 doi:10.1111/pai.13323.

    PMID: 32679608
  3. 3

    Early-onset familial HLH due to PRF1 mutation: diagnostic and therapeutic challenges in a resource-limited setting.

    Patel K, Kumar M, Paswan V, Prasad B

    BMJ case reports 2025; (18(12)) doi:10.1136/bcr-2025-266676.

    PMID: 41412946
  4. 4

    Diagnostic guidelines for familial hemophagocytic lymphohistiocytosis revisited.

    Henter JI, Sieni E, Eriksson J, et al.

    Blood 2024; (144(22)):2308-2318 doi:10.1182/blood.2024025077.

    PMID: 39046779
  5. 5

    Familial Hemophagocytic Lymphohistiocytosis Type 2 in a Chinese Infant with PRF1 Homozygous Mutation: a Case Report.

    Ji Q, Wang G, Xu W

    Clinical laboratory 2020; (66(7)) doi:10.7754/Clin.Lab.2019.191136.

    PMID: 32658436
  6. 6

    Gene-corrected human Munc13-4-deficient CD8+ T cells can efficiently restrict EBV-driven lymphoproliferation in immunodeficient mice.

    Soheili T, Rivière J, Ricciardelli I, et al.

    Blood 2016; (128(24)):2859-2862 doi:10.1182/blood-2016-07-729871.

    PMID: 27799161
  7. 7

    Accuracy of flow cytometric perforin screening for detecting patients with FHL due to PRF1 mutations.

    Abdalgani M, Filipovich AH, Choo S, et al.

    Blood 2015; (126(15)):1858-60 doi:10.1182/blood-2015-06-648659.

    PMID: 26450956
  8. 8

    Hemophagocytic Lymphohistiocytosis.

    Al-Samkari H, Berliner N

    Annual review of pathology 2018; (13()):27-49 doi:10.1146/annurev-pathol-020117-043625.

    PMID: 28934563
  9. 9

    Familial Hemophagocytic Lymphohistiocytosis Presenting as Hydrops Fetalis.

    Iwatani S, Uemura K, Mizobuchi M, et al.

    AJP reports 2015; (5(1)):e22-4 doi:10.1055/s-0034-1544110.

    PMID: 26199792
  10. 10

    The Safety and Efficacy of Ruxolitinib in an Infant With Familial Hemophagocytic Syndrome Type 3: A Bridging Therapy Toward Hematopoietic Cell Transplantation and Treatment of Post-Transplant Complications, Including Sinusoidal Obstruction Syndrome.

    Takahashi K, Hirabayashi S, Ueki M, et al.

    Pediatric blood & cancer 2025; (72(12)):e32073 doi:10.1002/pbc.32073.

    PMID: 40993979
  11. 11

    Haploidentical hematopoietic stem cell transplantation using reduced-intensity conditioning for pediatric patients with familial hemophagocytic lymphohistiocytosis.

    Jia C, Wang B, Zhu G, et al.

    Pediatric investigation 2018; (2(4)):216-221 doi:10.1002/ped4.12096.

    PMID: 32851268
  12. 12

    History of Hemophagocytic Lymphohistiocytosis.

    Janka GE

    Advances in experimental medicine and biology 2024; (1448()):9-19 doi:10.1007/978-3-031-59815-9_2.

    PMID: 39117804
  13. 13

    The clinical and laboratory evaluation of familial hemophagocytic lymphohistiocytosis and the importance of hepatic and spinal cord involvement: a single center experience.

    Beken B, Aytac S, Balta G, et al.

    Haematologica 2018; (103(2)):231-236 doi:10.3324/haematol.2017.178038.

    PMID: 29146706
  14. 14

    Brain MRI imaging markers associated with death in children with central nervous system involvement of hemophagocytic lymphohistiocytosis.

    Ma W, Zhou L, Li W, et al.

    European radiology 2024; (34(2)):873-884 doi:10.1007/s00330-023-10147-8.

    PMID: 37624411
  15. 15

    Histiocyte Society blueprint for hemophagocytic lymphohistiocytosis research: deciphering underlying disease mechanisms to optimize diagnosis and therapy.

    Meyer LK, Lee JC, Rocco JM, Nichols KE

    Haematologica 2025; (110(11)):2572-2587 doi:10.3324/haematol.2024.286477.

    PMID: 41178437
  16. 16

    Hemophagocytic lymphohistiocytosis in early pregnancy: A rare and fatal diagnostic challenge.

    Rojas-Suarez J, Ramos M, Lazarte I, et al.

    Obstetric medicine 2025; 1753495X251314168 doi:10.1177/1753495X251314168.

    PMID: 39867859
  17. 17

    Familial hemophagocytic lymphohistiocytosis in a girl with a novel homozygous mutation of STX11: A case report.

    Guo X, Jiang M, Tang X, Li Q

    Medicine 2019; (98(48)):e18107 doi:10.1097/MD.0000000000018107.

    PMID: 31770233
  18. 18

    FHLdb: A Comprehensive Database on the Molecular Basis of Familial Hemophagocytic Lymphohistiocytosis.

    Viñas-Giménez L, Padilla N, Batlle-Masó L, et al.

    Frontiers in immunology 2020; (11()):107 doi:10.3389/fimmu.2020.00107.

    PMID: 32076423
  19. 19

    Stem cell transplantation for children with hemophagocytic lymphohistiocytosis: results from the HLH-2004 study.

    Bergsten E, Horne A, Hed Myrberg I, et al.

    Blood advances 2020; (4(15)):3754-3766 doi:10.1182/bloodadvances.2020002101.

    PMID: 32780845

This guide is for informational purposes only and does not replace professional medical advice. Always consult your pediatric hematologist-oncologist about your child's FHL diagnosis, genetic testing, and treatment plan.

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