Rabbits immunized with killed type III pneumococci respond to anamnestic challenge by type specific polysaccharide (S III) with the synthesis of anti-S III antibody if a long interval is allowed to elapse between primary and secondary immunization. A study of the anti-S III antibody produced early and late in the immune response revealed no change in molecular class, banding pattern of dissociated light chains, or S III binding characteristics as measured under equilibrium conditions or by study of dissociation kinetics utilizing radioiodinated p-OH-benzyl-S III.

Sequential booster injections of S III into rabbits primarily immunized with whole organisms 8 or 9 months earlier led to a progressive decrease in the number of animals showing successful anamnestic responses and in the magnitude of those responses.

It is concluded that S III depletes the antigen sensitive cell population in the secondary response largely because of its limited ability to stimulate sustained proliferation by such cells.

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