Rh Factor: Are you positive or negative?
Ever pondered upon the + or – sign ahead of your blood
type?
Rh blood
types were discovered in 1940 by Karl Landsteiner and Alexander Wiener.
This was 40 years after Landsteiner had discovered the ABO blood groups.
The Rh system was named after rhesus monkeys, since they were initially
used in the research to make the antiserum for typing blood samples.
Rhesus protein is second in its clinical importance only to
the ABO blood group. The D antigen, discovered in 1939, was
the first Rhesus antigen to be described. D positive patients were termed
Rhesus-positive. In 1946, a quantitative variant with a weakly expressed D
antigen was discovered and termed “Du”. This variant, now called “weak
D”, is of clinical and diagnostic importance.
Since 1953, is has been clear that there are
also qualitative variants of the D antigen. Although patients with this partial
D variant are positive for the D antigen, they can also form anti-D. Up to 1% of all pregnant women have clinically
significant anti-erythrocyte antibodies.
Are you positive for
Rh?
Rh factor is a protein embedded in the membrane of red blood corpuscles.
Possessing this protein on the surface of RBC renders a positive phenotype and
lack of the same makes you negative. This can be detected simply by a test for
blood type and Rh status.
Rh Inheritance
Rh can be inherited from either of the parents, father or
mother, independent of blood type ABO alleles. It is transferred from the
parents to progeny through genes. There are two categories of Rh alleles, Rh
positive and Rh negative, former being the dominant. An individual positive for
Rh may have both alleles as Rh+/Rh+ or a copy of both Rh positive and Rh
negative Rh+/Rh-.
Rh factor and Pregnancy
Medically, the Rh factor and blood types can create serious
clinical complications. Commonest of all is mother-foetus incompatibility which occurs when wither the mother
is Rh (-) and foetus is Rh (+). Maternal antibodies can cross the placenta and
destroy the foetal red blood cells. Risk increases with each birth. Rh
type mother-foetus incompatibility occurs only when an Rh+ man fathers a child
with an Rh- mother. Since an Rh+ father can have either a DD or Dd genotype, there are 2 mating combinations possible with
differing risks as shown below. Regardless of the father's genotype, if
he is Rh+ and the mother is Rh-, doctors assume that there will be an
incompatibility problem and act accordingly.
If the blood of an Rh-positive foetus gets into the
bloodstream of an Rh-negative woman, her body will understand it is not her
blood and will fight it by making anti-Rh antibodies. These antibodies can cross the placenta and try to destroy the foetus’s blood. This reaction
can lead to serious health problems and even death in a foetus or new born.
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