PGM3 (phosphoglucomutase 3) catalyzes the conversion of GlcNAc-6-P to GlcNAc-1-P, a critical step in UDP-GlcNAc biosynthesis 1. This nucleotide sugar serves as a donor substrate for protein N- and O-linked glycosylation pathways essential for normal immune function. Hypomorphic PGM3 mutations cause autosomal recessive immunodeficiency 23, characterized by impaired UDP-GlcNAc biosynthesis and abnormal tri- and tetra-antennary N-glycan structures 2. Clinically, PGM3 deficiency presents as a hyper-IgE syndrome with recurrent infections, atopy, eczema, and elevated serum IgE levels 3. Mechanistically, PGM3 insufficiency impairs CD4+ T cell proliferation and differentiation in a dose-dependent manner, with reduced naïve CD4+ T cell counts and altered Th cell subset development favoring Th1/Th2 while attenuating Th17/Treg responses 4. Disease severity correlates with residual PGM3 enzyme activity 4. Beyond immune regulation, PGM3 controls hexosamine synthesis flux and regulates lipogenic transcription factor SREBP-1 5. The broad clinical variability in disease penetrance and expressivity suggests complex interactions with additional signaling pathways, possibly including IL-6 receptor signaling 6.