Recurrence of severe microbial infections results from a primary immunodeficiency disorder known as major histocompatibility complex class II deficiency or bare lymphocyte syndrome type II. Immunologic function is severely impaired due to the absence of MHC class II molecules on the surface of immune cells. Here, we report a 5-year-old boy with a novel homozygous mutation in RFXANK gene that negatively affects the proper expression of MHC class II molecules by antigen presenting cells. The frame shift mutations in RFXANK gene and negative HLA-DR proteins expression on peripheral blood mononuclear cells were identified and confirmed by whole exome sequencing, Sanger sequencing, and flow cytometry. The patient was referred with long-term severe prolonged diarrhea, fever, coughing, and vomiting. Also, antibiotic resistance, normal T cell, and NK cell counts with low B cell count and reduced serum immunoglobulin level were observed. The patient did not give a dramatic response to intravenous immunoglobulin infusion. The significancy of this report is the novelty of mutation and low B cell count which is not commonly expected in such patients. The final diagnosis of BLS type II is based on WES, Sanger sequencing, and flow cytometric evaluation. Moreover, it seems that the only therapeutic choice is hematopoietic stem cell transplantation.

严重微生物感染的复发是由原发性免疫缺陷病引起的，称为主要组织相容性复合体 II 类缺陷或裸淋巴细胞综合征 II 型。由于免疫细胞表面缺乏 MHC II 类分子，免疫功能受到严重损害。在这里，我们报告了一名 5 岁男孩，其 RFXANK 基因发生了新的纯合突变，该突变对抗原呈递细胞对 MHC II 类分子的正确表达产生负面影响。通过全外显子组测序、Sanger测序和流式细胞术鉴定并证实了RFXANK基因的移码突变和外周血单个核细胞上HLA-DR蛋白的阴性表达。该患者因长期严重腹泻、发烧、咳嗽和呕吐而被转诊。此外，抗生素耐药性、正常 T 细胞、观察到 B 细胞计数低和血清免疫球蛋白水平降低的 NK 细胞计数。患者对静脉注射免疫球蛋白没有显着反应。该报告的意义在于突变和低 B 细胞计数的新颖性，这在此类患者中并不常见。BLS II 型的最终诊断基于 WES、Sanger 测序和流式细胞术评估。此外，似乎唯一的治疗选择是造血干细胞移植。Sanger 测序和流式细胞术评估。此外，似乎唯一的治疗选择是造血干细胞移植。Sanger 测序和流式细胞术评估。此外，似乎唯一的治疗选择是造血干细胞移植。