[I antigenicity of the Bombay red cells]

Different homogeneous IgM cold agglutinins have been used to determine the antigen site density of Oh and OHm erythrocytes in comparison with O (H). The equilibrium constants and the delta Ho and delta S o have been measured. When H antigen is absent on the O red blood cells, there is no apparition of new I and i antigenic sites. Competition between gene products converting i into I and Hh genes for a common precursor substance appears unlikely. Nevertheless, the affinity of all antibodies tested increases with Oh and OHm cells and the thermodynamic parameters are accordingly modified. These results could be explained by: either a lesser steric hindrance in the absence of H antigen, the I determinants that are normally hidden by H antigen become more accessible, or by a modification of the molecular environment, as the interaction between fucosyl and N-acetyl residue of the penultimate sugar on the basic oligo-saccharidic chain is abolished. The structure of some I determinants may be modified, particularly of those which include the beta-D-Glu-NAc. Another explanation for these peculiar reactivities could be the substitution on the Bombay cells of this fucose by a neuraminyl residue. In the Sch. siblings, a heterozygous Hh subject was evidenced. His thermodynamic characteristics are nearing those of the hh propositus; the hypothesis of a competition between fucosyl and neuraminyltransferases for the same substrate is then proposed.