Two-chain disulphide-bonded structure of antigen-specific T-helper factor: both chains are necessary for activity and their interaction is I-A restricted.

The molecular structure of the antigen-specific T-helper factor (ThF) which augments contact sensitivity in mice was studied. ThF was split into two types of polypeptide chain by mild reduction and alkylation; one antigen binding (which determined the specificity), the other non-antigen binding. The two chains were, by themselves, inactive but complemented each other and reconstituted biological activity. In addition, a genetic restriction was observed in the complementation of the two chains which mapped to the I-A subregion of the H-2 complex.