Contribution of serotype-specific IgG concentration, IgG subclasses and relative antibody avidity to opsonophagocytic activity against Streptococcus pneumoniae

The contribution of serotype-specific IgG concentration, subclasses, and av idity to opsonophagocytic activity (OPA) against Streptococcus pneumoniae ( Pnc) was evaluated in sera of adults and infants immunized with different p neumococcal vaccines. Antibody concentrations and avidities were measured b y enzymeimmunoassay (EIA) and OPAs by killing assay of Pnc. The most import ant factor contributing positively to OPA was the specific IgG level. In in fants, a tendency to negative correlation was found between the concentrati on needed for killing of bacteria and avidity, suggesting that less antibod ies of high rather than low avidity were required for killing. No such corr elation was seen in adults. However, in adults the avidity was high already before vaccination and the variation was narrow. Thus, avidity was probabl y not a limiting factor influencing OPA. The effect of IgG2/IgG1 ratio on O PA was mostly negative but insignificant.