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Serum antibodies to PPS 4 in mice immunized intranasally with heat-inactivated pneumococci type 4, with CT (+CT; eight mice), or without CT (−CT; six mice). Immunizations (v) were carried out at weekly intervals (days 0, 7, 14, and 21), and blood samples were taken on days 0, 14, and 28. A control group (seven mice) was immunized intranasally with saline (NaCl). The symbols represent individual levels (absorbance) of IgG, IgM, and IgA antibodies.
