SUMMARY: Quantitative absorption of antisera was used to study the effect of calcium on the antigenic surface of Antisera to Ca-deficient and Ca-adequate rhizobia (whole or broken) revealed two parts in the absorption curve: in which there was ready removal of most of the agglutinating antibody with a very small absorbing dose; in which the remaining agglutinating antibody resisted absorption. When Ca-adequate bacteria were used for absorption, range I consisted of about 87% of the total titre. The corresponding figure with Ca-deficient bacteria was 95%. These values have been attributed to three types of antibody; avid I (readily absorbed by either kind of bacterium), avid 2 (readily absorbed by Ca-deficient bacteria), non-avid (difficult to absorb with either bacterium). The fact that avid antibody 2 was absorbed readily by Ca-deficient bacteria but with difficulty by Ca-adequate bacteria may be due to a quantitative deficiency of a particular antigen on the surface of the Ca-adequate bacterium, or to a structural condition which gives the antigen lower affinity for its homologous antibody. Absorption characteristic of Ca-deficient rhizobia was obtained with the Ca-adequate bacteria treated with EDTA under conditions known to remove 90% of Ca from the cell. Broken bacteria to some extent simulated the absorption curve found with Ca-deficient bacteria. It is suggested that Ca located in the surface lipopolysaccharide layer of the wall of rhizobium grown with a sufficiency of this element obscures or modifies an antigenic group. Glucuronic acid found in the somatic antigen fraction of this bacterium is suggested as a possible site of Ca action.


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