The observed increase of IgG1 and IgG2a in i.v. vaccinated mice after the challenge infection was also reflected in a strong shift in isotype proportions to IgG1 (47% vs. 67% of the total Ig), and a concomitant decrease in IgG3 (20% vs. 12%) and, as expected, IgM (19% vs. 7%). This evidence concerns the gene IGHG3 and infection.