Although IgG2 is a poor activator of the classical complement cascade, effective opsonisation and killing of certain bacterial strains and protection in bioassays by purified IgG2 have been reported. This may be explained by a combined effect of both bacterial antigen and IgG2 on the activation of complement. Epitope density and accessibility of antigenic determinants on the bacteria may determine the actual complement activation by IgG2. Complement activation via the alternative route appears to be especially important in the opsonisation and killing of bacteria. Most micro-organism are by themselves incapable to activate complement or to bind phagocytes.
Antibodies act as flexible adapters mediating the adherence of infectious agents to phagocytes.
The antibody forms a bridge to attach the microbe to the Fc receptor on the phagocyte membrane. Interaction with Fc receptors Phagocytosis is initiated by an interaction between the Fc fragment of the immunoglobulin and receptors. The four human IgG subclasses show differences in their interaction with Fc g R's. The latter comprise e.g. phagocytosis, endocytosis, antibody-mediated cellular cytotoxicity, release of a range of inflammatory mediators, antigen presentation and clearance of immune complexes. Since IgG subclasses differ in their affinity to Fc g R, clinical manifestations of IgG subclass deficiencies will depend upon the properties of the Fc g R as well.