and humoral immunity is the principal mechanism of defense against polysaccharide-rich encapsulated bacteria. The effector mechanisms used by antibodies to combat these infec- tions include neutralization, opsonization and phago- cytosis, and activation of complement by the classical pathway (see Chapter 13). Neutralization is mediated by high-affinity IgG, IgM, and IgA isotypes, the latter mainly in the lumens of mucosal organs. Opsonization is mediated by some subclasses of IgG, and complement activation is initiated by IgM and subclasses of IgG.