Counter-inhibition assay used to identify canonical Ab of switched specificity. A, Binding of biotin-labeled Ab 3A4 to anti-Id E4 is inhibited by canonical E4+ Id in serum. If serum Id binds hapten, inhibition is reversed. Note, 3A4 does not bind Ars-Tyr or Sulf-Tyr. B, Representative results for two sera, one of which contains E4+ Id that binds more strongly to Sulf-Tyr than Ars-Tyr (switched specificity, representative of group #5 Table IV), and the other of which behaves conversely (not switched, representative of control group Table IV).