Haemophagocytic lymphohistiocytosis:
a diagnostic challenge for the pediatrician

Key words: Haemophagocytic lymphohistiocytosis, Cellular cytotoxicity, Natural killer, Haemophagocytosis

Haemophagocytic lymphohistiocytosis (HLH) is a severe hyperinflammatory syndrome caused by uncontrolled but ineffective immune response. Inherited human disorders causing the defect of selected proteins involved in the cellular cytotoxicity machinery are responsible for this disease. Since 2000 some genes, whose mutations cause the disease, have been progressively identified. Occasionally a transient state of functional impairment may result from a contingent situation; these patients without evidence of a genetic defect are defined as affected by “secondary” or “acquired” HLH. The international cooperative effort has allowed to develop, by prospective trials, an effective therapy that today enables most affected children to be cured. A diagnostic strategy based on clinical criteria and functional assays has also been defined, thus providing a strong support to the clinical diagnosis and directing early start of specific therapy. On this basis, genetic diagnosis can now be directed to the gene(s) most likely involved; definitive diagnosis allows to define indication to haematopoietic stem cells transplantation, familial genetic counselling, with prenatal diagnosis. Investigation of these rare genetic cases provided important clues for better understanding some mechanisms of the immune system in humans.