AbstractMeasurements have been made of the number of available sites on 10 examples of red cells in which the only abnormality appeared to be a quantitative reduction in the expression of D (weak D cells); these estimates were carried out using three monoclonal anti‐D antibodies, Fog‐1, Brad‐3 and Los‐2. The values varied with the monoclonal antibody that was used and fell within the range of 170–1,870 sites/cell. A further 3 examples of weak D cells which had brought about immunisation following transfusion were found to have between 390 and 1,470 sites per red cell. The implications of the D site density on the immunogenicity of weak D cells are discussed. The number of sites on red cells with structurally abnormal D (partial D cells) were also estimated, using the antibody Fog‐1. Four of the 5 examples of cells of category IVa (probable phenotype R0r) were found to have a high expression of D (range 29,300–41,300), but the available D sites of categories DVa, DVIa, and DVIIwere considerably reduced (<500,<500 and 2,400–7,500 sites/cell, respectively). As a working hypothesis, it is suggested that there are two types of genetic abnormality leading to an abnormal expression of D. First, a defect in genomic DNA leading only to a quantitative reduction in the number of available D sites; this genomic lesion should be termed ‘weak D’. Secondly, genomic defects leading to amino acid sequence abnormalities and structural change in the D polypeptide; these lesions should be collectively k
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