The immunoglobulin G (IgG) molecule contains two biantennary complextype oligosaccharide chains, each linked to the heavy chain at asparagine 297 within the CH2 domain (1) (see Note 1). X-ray crystallographic analysis suggested that the sugar chains of IgG play a role in maintaining the 3D structure of its Fc portion by bridging the two CH2 domains (2-4). Although these sugar chains can possess the complete structure shown in Fig. 1, normally only 25% of the sugar chains are sialylated, which is unusual because the sugar chains of other serum glycoproteins are highly sialylated Also characteristic is the extremely high microheterogeneity resulting from the presence or absence of the two galactose (Gal), the bisecting N-acetylglucosamine (GlcNAc), and the fucose (Fuc) residues (1). Fig. 1. Structure of asparagine-linked sugar chain of human IgG.