IGHG3 and pneumococcal infection: Immunization with TI-2 Ags, including NP-Ficoll and bacterial polysaccharides, predominantly elicits IgG3 (Perlmutter et al., 1978; Slack et al., 1980; Rubinstein and Stein, 1988) and IgG3 is required for protection against pneumococcal infection (McLay et al., 2002).