Tehran University of Medical Sciences Iranian Journal of Allergy, Asthma and Immunology 1735-1502 0 0 2026 07 22 1 7 Mehrsa Hamidi Students' Scientific Research Center, Tehran University of Medical Sciences, Tehran, Iran Zohreh Habibi Department of Neurosurgery, Children’s Medical Center, Tehran University of Medical Sciences, Tehran, Iran Nima Parvaneh Division of Allergy and Clinical Immunology, Department of Pediatrics, Children’s Medical Center Hospital, Tehran University of Medical Sciences, Tehran, Iran Mahshid Movahedi Division of Allergy and Clinical Immunology, Department of Pediatrics, Children’s Medical Center Hospital, Tehran University of Medical Sciences, Tehran, Iran 2026 02 27 2026 04 30 Wiskott–aldrich syndrome (WAS) is a rare X-linked primary immunodeficiency characterized by thrombocytopenia, eczema, and recurrent infections. Live vaccines, such as Bacillus Calmette–Guérin (BCG), routinely administered in tuberculosis-endemic regions, may cause severe complications in undiagnosed patients. We report a 2-year-old boy with genetically confirmed WAS who developed a rare BCG-related brain abscess. The patient initially presented with neonatal thrombocytopenia and eczema, followed by recurrent respiratory and gastrointestinal infections. Whole exome sequencing identified a pathogenic mutation in the WAS gene. During pre-hematopoietic stem cell transplantation (HSCT) evaluation, brain imaging revealed a large multiloculated right parieto-occipital abscess with significant mass effect. Surgical drainage was performed, and polymerase chain reaction confirmed Mycobacterium bovis (BCG strain). Antitubercular therapy was initiated. Severe thrombocytopenia complicated management and necessitated splenectomy. Although platelet counts improved and infection was partially controlled, HSCT was postponed due to active infection. The patient subsequently died following a severe COVID-19 infection. This case highlights the diagnostic and therapeutic challenges of BCG-related brain abscess in patients with WAS and underscores the importance of early genetic diagnosis, avoidance of live vaccines, and timely definitive treatment in primary immunodeficiencies. https://ijaai.tums.ac.ir/index.php/ijaai/article/view/4767 https://ijaai.tums.ac.ir/index.php/ijaai/article/download/4767/2373