The N-linked sugar chains of human immunoglobulin G: their unique pattern, and their functional roles.

In contrast to other serum glycoproteins, the majority of the N-linked sugar chains of human serum IgG are not sialylated. In addition, extremely high micro-heterogeneity occurs in the serum IgG sugar chains. This micro-heterogeneity is mainly produced by the presence or absence of the two galactoses, the bisecting GlcNAc, and the fucose residue. Interesting evidence is that the molar ratio of each sugar chain of the serum IgG samples is quite constant in healthy individuals. By adding the information of the characteristic feature of the sugar patterns of myeloma IgG samples and glycosylated Bence Jones proteins, which are the products of monoclonal B-cells, it was proposed that B-cells in the human blood are a mixture of clones equipped with different sets and ratios of glycosyltransferases. It was also proposed that each glycoform of IgG might have a different function. This hypothesis was realized by the comparative studies of the function of IgG samples before and after removal of galactose residues, fucose residue, or sialic acid residues.