Familial Hemophagocytic Lymphohistiocytosis.

Familial hemophagocytic lymphohistiocytosis (FHL) is a rare heritable disorder of immune regulation that is typically characterized by sudden onset of severe systemic illness. Functional impairment or absence of 1 or more of several proteins that participate in lymphocyte cytotoxicity underlies the disease. Although FHL usually presents in infancy, age of onset is variable and dependent on genetic and environmental factors. Initial treatment consists of immune suppression, whereas definitive treatment requires hematopoietic cell transplantation.