The modification of hemoglobin-antihemoglobin reaction by haptoglobin

In radioimmunoassay and double diffusion experiments, the reaction between hemoglobin (Hb) and antihemoglobin antibody is markedly affected by the presence of haptoglobin (Hp). Binding with haptoglobin exposes "neoantigenic" determinants in hemoglobin, probably as a result of conformational changes exposing previously inaccessible structures. Some of these are shared by both Hb A and Hb F an may be located on alpha-chains. The exposure of these determinants requires the binding of hemoglobin by haptoglobins of the phenotype Hp 2-2 or Hp 2-1. The phenotype Hp 1-1 and guinea pig haptoglobin have no discernible effect in this context. It thus appears that the conformational change induced in hemoglobin by the "polymeric" forms of haptoglobin differ from those, if any, caused by the "monomeric" forms such as Hp 1-1. Most antisera raised against free hemoglobin contain some antibodies specific for these hidden determinants which may have become exposed during the processing of the immunizing antigen. These findings have significant implication in the interpretation of radioimmunoassay data.