The presence of sialic acid on two related bacterial polysaccharides determines the site of the primary immune response and the effect of complement depletion on the response in mice.

We have examined the antibody responses in mice to two structurally similar antigens: the capsular polysaccharide of type 3 group B streptococci (sssGBS 3) and the capsular polysaccharide of type 14 pneumococci (sssPn 14), which differ only in the presence of a terminal sialic acid on the side chain of the former. The cells that produce antibody to the nonsialated antigen (sssPn 14) reside in the spleen, whereas the cells that produce antibody to the sialated antigen (sssGBS 3) do not. Cobra venom factor treatment of the mice before immunization abrogates the antibody response to the nonsialated antigen, but does not affect the response to the sialated antigen.