X-Linked Inhibitor of Apoptosis (XIAP) deficiency presenting as hemophagocytic lymphohistiocytosis triggered by drug allergy

X-linked inhibitor of apoptosis (XIAP) deficiency is a primary immunodeficiency associated with recurrent hemophagocytic lymphohistiocytosis (HLH) episodes. The clinical phenotypes of XIAP deficiency vary, ranging from splenomegaly to life-threatening inflammation. We report a case of XIAP deficiency with unusual late-onset HLH presentation likely triggered by a drug allergy. A previously healthy adolescent boy presented to the hospital with fever and rash seven days after starting antibiotics for a neck abscess. Laboratory evaluation demonstrated cytopenias, elevated liver enzymes, and increased inflammatory markers. Initially, antibiotics were discontinued due to concern for drug rash. He continued to deteriorate clinically and became hypotensive. Additional testing revealed decreased NK cell function, as well as elevated ferritin, triglycerides, and soluble IL-2 receptor. SLAM-Associated Protein (SAP) and XIAP evaluation by flow cytometry demonstrated decreased XIAP expression. Subsequently, genetic testing revealed a known pathogenic mutation in BIRC4 (c.421_422del), confirming the diagnosis of XIAP deficiency.