EBV-Driven HLH and T Cell Lymphoma in a Child with X-Linked Agammaglobulinemia: A Genetically Confirmed Case Report and Literature Review

<b>Introduction</b>: X-linked agammaglobulinemia (XLA) is a prototypical inborn error of immunity (IEI) caused by mutations in the <i>BTK</i> gene, leading to a profound deficiency of mature B cells and severe pan-hypogammaglobulinemia. The Epstein-Barr virus (EBV), which primarily infects B lymphocytes, is believed to be unable to establish persistence in these patients due to the lack of its natural reservoir. Indeed, current evidence supports that EBV infection is typically refractory in individuals with XLA. <b>Methods</b>: We describe the clinical and molecular characterization of a 10-year-old male patient with genetically confirmed XLA who developed EBV viremia, hemophagocytic lymphohistiocytosis (HLH), and EBV-positive cutaneous T cell lymphoma. Diagnosis was supported by flow cytometry, serology, quantitative PCR, EBER <i>in situ</i> hybridization, histopathology, and whole-exome sequencing. <b>Results</b>: Despite the complete absence of peripheral B cells, EBV was detected in leukocytes and multiple tissues, indicating active infection. The patient developed HLH and a T cell lymphoma with EBER-positive infiltrates. Genetic analysis revealed a nonsense mutation in <i>BTK</i> (1558C>T, R520*), confirming XLA. The clinical course included multiple episodes of neutropenia, viral and bacterial infections, and severe systemic inflammation. <b>Conclusions</b>: This is the first documented case of an XLA patient with confirmed <i>BTK</i> mutation presenting with clinical features more consistent with chronic active EBV infection. These findings challenge the prevailing paradigm that XLA confers protection against EBV-related diseases and further support the possibility of EBV noncanonical reservoirs leading to immune dysregulation. EBV should also be considered in the differential diagnosis of XLA patients presenting with systemic inflammation or lymphoproliferative disease.