The response of inbred mouse strains to two polypeptides derived from multichain polyprolines, (T,G)-Pro--L and (Phe,G)-Pro--L, is different from the response of the same mouse strains to a similar series of polymers built on multi-poly-D,L-alanyl--poly-L-lysine, although the same short sequences of amino acids are attached to the side chains of the polypeptides in the two series. These results indicate that a portion of the side chain (e.g. polyalanine or polyproline) participates in the antigenic determinant. This was confirmed by studying the response of different mouse strains to two kinds of polypeptides: (T,G)-Pro-A--L 717 and 718 and (T,G)-A-Pro--L 719 and 721.

Antibody assay of antisera to (Phe,G)-Pro--L with the cross-reacting antigens (T,G)-Pro--L and (Phe,G)-A-L indicates that different inbred mouse strains make antibodies specific for different parts of the same polypeptide. Thus, antibody from DBA/1 mice reacts almost exclusively with the (Phe,G) sequence, while SJL antisera bind only (T,G)-Pro--L and fail to bind (Phe,G)-A-L.

The immune responses to the same amino acids on two different polypeptides (i.e. A--L and Pro--L) appear to be under separate genetic control.

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