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Immunity in Infection and Vaccination
James Chin, Immunology, CVL, GLENFIELD, NSW
Although the existence of 3 major classes of ruminant immunoglobulins (Igs) - IgG, IgA and IgM have been known for some time, a functional role for these molecules based on compositional differences and structural diversity has not been advanced. The importance of structure and function is implicit in ascribing discrete roles to each Ig subclass as illustrated by the importance of IgA in mucosal immunity, while IgG and IgM appear to dominate systemic humoral immunity.
Data is presented which show that the carbohydrate content of Ig subclasses can be ranked as follows: IGM > IgA > IgG. While IgM has the greatest content of carbohydrate molecules, its structure is such that these sugar groups are primarily sequestered. In contrast, sugar groups in IgA are more exposed and hence IgA is the most hydrophobic immunoglobulin. IgG is at the other end of the scale and is considered to be extremely hydrophobic.
Conceptually, the degree of hydrophobicity attributed to IgA explains why it has an important role in mucosal immunity. Invading organisms coated with IgA become hydrophobic and cannot be readily phagocytosed. Rather, IgA coated organisms are prevented from binding to epithelial surfaces. They are therefore flushed away with mucilaginous secretions. In this sense, IgA has an aseptic role. IgG and IgM function quite differently since coating of microbes with these molecules confer comparatively greater hydrophilicity. Systemically, IgG and IgM coated pathogens attract complement activity and become extremely palatable to phagocytes.