Serological cross-reactivity between products of separate I regions.

A B10.S(7R) anti-B10.S(9R) serum (anti-IJEkCd) contained, as expected, antibodies specific for the I-E-subregion-encoded determinant Ia.7. However, tests on recombinant haplotypes demonstrated a series of unexpected weak extrareactions which could be interpreted to be directed against antigenic determinants encoded in the I-A subregion of the H-2 complex. The same type of extrareaction was observed in eluates from I-As, I-Ek cells coated with A.TH anti-A.TL (I-As, I-Ek anti-I-Ak, I-Ek) serum. This reactivity in serum and eluates could be interpreted as cross-reactivity between products of the I-E and I-A subregions.