(3) In some heterozygous individuals, the Du phenotype seems to result
from suppression of a perfectly normal gene by the other allele. The gene r' (Cde)
sometimes affects the expression of Rh-positive genes present on the chromosome,
TRANS. (This is known as TRANS positioning.) When this occurs, the cell may have
weakened Rho(D) activity, but it is usually stronger than on cells whose Du is controlled
by a specific gene. This phenotypic trait is not transmitted genetically, and not every
Rh-positive individual with a r' (Cde) gene will show this effect.
b. Subunits of Rho(D) - Dmosaic. Observations that the Rho(D) antigen includes
numerous genetically determined subunits explained the behavior of some Du bloods
and some hitherto inexplicable cases of Rh-positive persons producing anti-Rho(D).
Wiener and Unger have shown that normal Rh-positive cells include all the subunits, to
which they give the designation Rho associated cognate specificities: RhA, RhB, RhC,
and RhD; in rare examples of Rh-positive blood, 1 or more cognate specificities may be
absent. Some cells lacking aspects of the Rho(D) spectrum may react weakly with anti-
Rho(D), and be classified as Du. As many as 50 percent of Du bloods may have this basis.
Other deficient cells may give unremarkable Rh-positive reactions, but if such patients
receive Rh-positive blood, they will be exposed to an antigen absent from their own
cells. Should they produce antibody to the missing subunit, it would react with nearly all
Rh-positive cells but their own, and would appear to be anti-Rho(D). Tippett has
classified such persons into six numbered groups. Although arrived at by different
criteria, the classifications of Race and Sanger and Wiener need not be incompatible.
c. Significance of Du.
(1) Since Du cells are Rh-positive, it is important that they not be given to
Rh-negative recipients. Although much less antigenic than standard Rh-positives, such
cells are capable of eliciting an anti-Rho(D) antibody, and if transfused into an Rh-
negative patient who already has an antibody, they may suffer accelerated destruction.
All donor bloods must be fully tested to exclude Du reactivity before they are classified
(2) More controversy exists about the status of the Du transfusion recipient.
Theoretically, such a patient, being Rh-positive, can receive Rh-positive blood with
impunity. Some workers believe that those individuals whose Du is a result of missing
subunits have increased risk of developing antibodies, which, although directed against
a particular cognate specificity, are effectively anti-Rho(D). This happening is rare. In
some transfusion centers, Du recipients are routinely given Rh-positive blood. A
different consideration influences other workers, who fear that careless or incorrect
interpretation of the Du test might lead some Rh-negative patients to be incorrectly
typed as Du and thus inappropriately to receive Rh-positive blood. The AABB Standards
requires that donor bloods, but not recipient bloods, be tested for Du. Many workers
believe that giving Rh-negative blood routinely to a patient whose cells are not
immediately agglutinated by anti-Rho(D) is the safest, most efficient, and most
economical way to handle this issue.