Studies of the relative combining capacities of different antisera to Vibrio cholerae and Shigella flexneri were carried out using the Talmage test. In this test the absorption of an I131 labelled antibody preparation by antigen is blocked by the addition of unlabelled unknown serum.

Sera from rabbits in the early stages of immunization ("early antisera") had a high agglutinin titer, but low combining capacity. Sera from hyperimmune rabbits ("late antisera") had higher combining capacities than early antisera, but similar or lower agglutinin titers. More antigen was needed to absorb the agglutinins from late antisera than from early ones of the same agglutinin titer, indicating that late antisera contained relatively more antibody. At high concentrations, sera from hyperimmune rabbits agglutinated the homologous antigen more rapidly than did early antisera, even though early sera had similar or higher agglutinin titers. Sera drawn after the anamnestic response to bacterial antigen had the characteristics of late sera; i.e., their combining capacities were high.

The possible significance of these findings for the in vitro estimation of protective antibody is discussed.

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