Measurements have been made of the number of available sites on 10 exa
mples of red cells in which the only abnormality appeared to be a quan
titative reduction in the expression of D (weak D cells); these estima
tes 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 follow
ing 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 str
ucturally abnormal D (partial D cells) were also estimated, using the
antibody Fog-1. Four of the 5 examples of cells of category IVa (proba
ble phenotype R(o)r) were found to have a high expression of D (range
29,300-41,300), but the available D sites of categories D(Va), D(VIa),
and D(VII) were considerably reduced (<500, <500 and 2,400-7,500 site
s/cell, respectively). As a working hypothesis, it is suggested that t
here are two types of genetic abnormality leading to an abnormal expre
ssion of D. First, a defect in genomic DNA leading only to a quantitat
ive 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 known as 'partial D'.