Tehran University of Medical Sciences Iranian Journal of Allergy, Asthma and Immunology 1735-1502 17 6 2018 12 04 594 600 EN Farhad Abolnezhadian Department of Pediatrics, Abuzar Children’s Hospital, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran Ali Saeedi-Boroujeni Student Research Committee, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran and Department of Immunology, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran and Abadan School of Medical Sciences, Abadan, Iran Sara Iranparast Student Research Committee, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran and Department of Immunology, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran 2017 11 11 2018 07 31 Major histocompatibility complex (MHC) class II deficiency is a rare primary immunodeficiency disorder (PID) with less than 200 cases worldwide. Here, we report an 8 month–old girl with MHC class II deficiency with a novel homozygous mutation in RFXANK gene (NM_001278728: exon 5: c.495G>A: p.Trp165*) and normal CD4+ T cell counts, diagnosed by whole exome sequencing (WES) and negative HLA–DR proteins on peripheral blood mononuclear cell (PBMC) in flow cytometry. She was referred with pneumonia, prolonged fever, resistance to antibiotics (ceftriaxone, clindamycin, and vancomycin), and low serum immunoglobulin (IG) levels, while natural killer (NK), B, and T cells were normal. She received intra-venous immune-globulin (IVIG) replacement, broad spectrum antibiotics, and anti-fungal treatments. The presented case report is interesting not only because of the rarity of the PID but also due to normal CD4+ T cell counts. According to our experience, we suggest that physicians consider MHC class II deficiency in families with consanguineous marriages, even with normal CD4+ T cell counts. At the first, the diagnosis of the disease could be successfully perform using WES, and finally, treatment with hematopoietic stem cell transplantation can save the patients’ lives. https://ijaai.tums.ac.ir/index.php/ijaai/article/view/1726 https://ijaai.tums.ac.ir/index.php/ijaai/article/download/1726/1383