The I blood group system in humans was investigated in order to study cell surface changes in development.
One advantage in studying the I-antigen in man is the availability of well defined, easily purifiable cold agglutinin molecules with anti-I specificity. Sera from patients with chronic cold agglutinin, post pneumonia cold agglutinin and post viral influenza cold agglutinin disease have been studied. The IgM agglutinins were isolated and their restricted heterogeneity established. These well characterized protein molecules were used in experiments to study the I antigen's development and in experiments designed to reveal information about the molecular basis of I antigen specificity.
The development of I antigen and postnatal changes in hemoglobin were examined in human infants, to see whether the developmental changes in these two attributes are correlated individual cells in such a way as to suggest that they have common biochemical control mechanisms. The results demonstrated that the expression of I antigen on the erythrocyte surface is independent from the control mechanism of the biosynthesis of the beta chain of hemoglobin. These observations are explained by suggesting two separate modulatory regulations, one for the enzymes involved in the expression of I-specific molecules on the cell surface and another for the regulation of hemoglobin subunit synthesis.
The rates of A-antigen and I antigen site development were compared using I125 labeled anti-A IgG molecules in order to test for possible common control or close association. Erythrocytes from postnatal infants were fractionated with respect to their I-agglutinability. Experimental results indicate that cells with well expressed I-specificity have more A-antigen sites; less sites were found on erythrocyteswith weak expression of I antigen. These results suggest that enzymes responsible for the expression of these two antigenic specificities are under a common control mechanism or that they are closely associated.
Erythrocyte stromata from human adults and from umbilical cord blood samples were fractionated. Fingerprints of the two membrane preparations indicated that the major protein components are identical. Molecular fractionation of the stomata resulted in fractions with I antigen activity. All fractions with I-activity contained ABO blood activity, but some preparations with ABO activity failed to inhibit agglutinins with I-specificity. Hydrophilic fractions containing glycopeptides with blood group activity were isolated.TheI-activity was pronase sensitive. Quantitation of molecular fractions with I blood group activity from cord and adult erythrocytes suggests that I-negative phenotype could be explained by either the two dimensional distribution of molecules on the cell surface or by the covering up of theI-specific molecules in the membrane.
The significance of these findings are discussed with respect to embryonic cell surface specificity and with respect to the recognition of molecular patterns by IgM molecules with multiple binding sites.
