Serological analysis of antigen‐specific helper factors specific for poly‐L(Tyr, Glu)‐poly‐DLAla–poly‐LLys [(T, G)‐A–L] and LGlu⁶⁰‐LAla³⁰‐LTyr¹⁰ (GAT)

In vitro prepared antigen‐specific helper factors reactive to the synthetic polypeptide antigens poly‐L(Tyr, Glu)‐poly‐DLAla‐ ‐poly‐LLys [(T, G)‐A‐ ‐L] or LGlu⁶⁰‐La³⁰‐ LTyr¹⁰ (GAT) and bearing Ia determinants were analyzed serologically to determine the nature of the Ia determinants they expressed. I subregion‐specific mouse anti‐Ia antisera were used, and showed that (T, G)‐A–L specific helper factor (HF) contains I‐A subregion‐controlled determinants, whereas GAT‐specific HF carries I‐J subregion‐controlled antigens. This unexpected finding was confirmed in both the H‐2^k and H‐2^b haplotypes, using a variety of anti‐I‐J antisera. Rabbit anti‐Ia antisera also reacted with both HF which raised the possibility that the Ia determinants on HF may be carbohydrate in nature. The fact that HF has a low molecular weight and yet contains Ia determinants, antigen‐binding capacity and idiotypic markers is compatible with this interpretation.