MGAT4A (alpha-1,3-mannosylglycoprotein 4-beta-N-acetylglucosaminyltransferase A) is a key enzyme that catalyzes the formation of a specific branch on N-linked glycans (the transfer of β1,4-GlcNAc to the α1,3-mannose arm), belonging to the glycosyltransferase family. It is responsible for trimming and remodeling N-glycans in the Golgi apparatus to generate tissue- and disease-specific N-glycan structures. MGAT4A and MGAT4B are isoenzymes that catalyze the transfer of GlcNAc via a β1-4 linkage to the α1-3 linked mannose of the N-glycan core, thereby regulating glycoprotein maturation and function. This enzyme is involved in immunoglobulin G glycosylation, insulin receptor signaling, and neural cell adhesion molecule function. Deficiency of MGAT4A leads to decreased IgG galactosylation and enhanced pro-inflammatory activity, which is associated with an increased risk of autoimmune diseases such as rheumatoid arthritis and IgA nephropathy. Concurrently, it affects insulin secretion in pancreatic β-cells, highlighting its dual role in metabolic regulation and immune homeostasis.