The nature of two distinct anti-pneumococcal antibodies of mice has been further investigated. These antibodies, produced after the injection of purified SIII (the capsular polysaccharide of Diplococcus pneumoniae Type III), were examined for their serological properties in vitro and for their protective potency in vivo.
The first anti-SIII antibody, distinguished by its ability to agglutinate antigen-coated erythrocytes, was not only unable to lyse these erythrocytes (with complement), but it protected them against lysis by the other antibody, because of its high affinity for the antigen. This haemagglutinating antibody did not precipitate soluble antigen, was of the IgA class, and could protect mice against challenge with virulent pneumococci. It was produced by mice given small doses (0.1 μg) of SIII, but not by mice given 50 μg. These latter were susceptible to pneumococcal challenge.
The second antibody, which was haemolytic and precipitating but not haemagglutinating, had a comparatively low affinity for the antigen: thus it was not detectable when mixed with the first antibody, especially if the test erythrocytes were inadequately sensitized. This haemolytic antibody was of the IgM class. It was found in high titre in mice given 50 μg of SIII, but did not protect them against challenge.