The tests presented in Tables I, II, and III show that only in the higher concentration cross reactions do take place, and that there is definite specificity of the two sorts of immune sera for the homologous antigens. Thus it is easy to differentiate the l- and d-antigens in dilutions 1:100 and upwards. The occurrence of cross reactions can readily be ascribed to the fact that the l- and d-acids present in the two antigens are identical in every respect but the position of the groups connected with the asymetric carbon atom. The i-antigen reacts with both sorts of immune sera as could be expected since it must consist of a mixture of equal parts of l- and d-antigen.

The reactions of the i-antigen appear to be only slightly weaker than those of the homologous ones owing to the fact that the intensity of the reactions diminishes but slowly with increasing dilution of the antigens. It is also to be considered that small differences cannot be judged very accurately.

Tests with two l- and four d-immune sera not recorded in the tables confirmed the results already discussed.

Considering that ferments are known to be adapted ordinarily to one type of steric isomers it may be worth noting that antibodies were formed by the same species of animals for optical antipodes.

From the results summarized in Tables IV,a and IV,b one sees that the l- and d-immune sera also differentiate clearly between the l- and d-acids when they are not diazotized and not combined with protein. The l-acid inhibits much more the precipitation of the l-antigen by the homologous immune serum than the d-acid and the converse effect occurs if the inhibiting action is tested on the precipitation of d-antigen by d-immune serum. The inactive phenyl (para-aminobenzoylamino) acetic acid behaved in such tests as a mixture of l- and d-acids, i.e., it acted markedly in both cases, more than the heterologous and less than the homologous acid.

The experiments reported bring a definite proof for the view that the steric configuration of antigenic groups is one of the factors determining serological specificity. In the particular case under consideration the mere difference in the position of H and COOH as indicated in the following formulas sufficed to alter the reactivity.

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The fact that steric isomers are acted upon selectively by immune sera may be supposed to play a significant part in the serological specificity of carbohydrates such as those discovered in bacterial antigens.

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