Glycan Structures of Human Immunoglobulins and Their Roles

Immunoglobulins (Igs) are antibodies that specifically recognize foreign bodies and induce immune responses. Among the five classes of Igs in humans, IgG, IgA, IgM, IgD, and IgE, human IgG is one of the most intensively studied glycoproteins because of its central role in the immune system as a major serum antibody and its effectiveness in clinical applications. The three-dimensional structures of human IgG with glycosylation suggest that subtle differences in glycan structures affect the protein structure of IgG, leading to significant differences in immune responses. To generate more effective antibody medicines, glycoengineering techniques have recently made it possible to manipulate antibodies with only specified N-glycan structures on the Fc region of IgG. In addition to the human IgG, the glycan structures and site occupancies at glycosylation sites on other Ig classes have also been investigated in detail by glycomics/glycoproteomics analysis using mass spectrometry. Human IgA and IgM are also expected to be used as therapeutic antibodies due to their unique properties and functions in the immune system. Site-specific glycan structures on IgD and IgE from a single donor can be analyzed using highly sensitive glycoproteomics, regardless of their relatively small amount in human serum. Although N-glycans of each Ig class often have similar structures, the degree of their contributions to the interaction with Fc receptors varies among the classes.