All of us bleed red – but of various types. It is common knowledge that blood can be classified
into different blood
groups. Those of us who are a part of any social network that provides a conduit for blood to
flow from person to
person, know that the following blood groups (or types) exist: A, B, AB, and O. Also familiar to
us, is the positive or
negative sign that accompanies these basic blood types, further segregating them into eight
groups. On what basis, then,
are these divisions organised?
The ABO grouping system is based on the type of antigens that live on the surface of the red
blood cells (RBCs). If the
word ‘antigen’ rings a rusty bell, we may recollect that these are protein molecules that
stimulate an immune response
from the white blood cells (WBCs) in the body. These need not necessarily be foreign bodies: our
systemgenerates
‘autoantigens’ on its own as well. The type of autoantigens that our RBCs carry determines our
blood group in the
following manner:
> Blood group A has type A antigens (and type B antibodies)
> Blood group B has type B antigens (and type A antibodies)
> Blood group AB has type A as well as type B antigens (and no antibodies)
> Blood group O has no antigens at all (and both types of antibodies)
> (Antibodies are also protein molecules launched by the body to counter the effect of
antigens.)
Rh Blood Types
A further classification is possible based on the Rh (Rhesus) factor. Named after the Rhesus
monkey that was mistakenly
thought to have the same antigen, this protein – which is also an antigen – is found on the
cell membrane of RBCs. The
determination of the various types within the Rh factor itself is a complex science, but as
far as blood groups are
concerned, the classification is based, quite simply, on their presence or absence. Blood
that lacks Rh antigens is
Rh-negative, and the other kind is Rh-positive. Accordingly, a plus or a minus sign is
attached to the ABO
classification, giving rise to these types: A+, A-, B+, B-, AB+, AB-, O+, and O-.
What is the significance of blood typing?
The problem and the answer both lie in the antigens. Since these substances trigger immune
responses from the body, it
is of utmost importance to ensure compatibility before blood transfusions and donations are
made. If, for instance, a
person of blood group B receives blood of type A, the recipient’s body will label the A-type
antigens as foreign bodies,
and resist the transfusion. It is for this reason that group AB can receive blood from any
type (since it contains and
recognises both types of antigens) but group O can only take from type O. The same rule
applies for the highly
immunogenic Rh-factor as well. Rh-negative mothers who first had an Rh-positive baby might
develop antibodies that may
harm an Rh-positive baby during the second pregnancy, leading to severe anaemia. It is not
surprising, therefore, that
many documents – ranging from university applications to employee ID-cards ask for the blood
group to be specified.
What is the testing procedure ?
The same principle mentioned above is applied in the laboratory.
In the first step, known as forward typing, antibodies that attack type A and type B blood
are introduced into the blood
sample, and the reaction is observed. If the target blood cells group together, it means
that the blood sample has
reacted with the antibodies. For instance, anti-A antibodies induce agglutination (grouping
or clumping) in type A
blood.
The second step is called reverse typing, and is performed on the serum left behind after
RBCs and WBCs are removed.
Blood of a predetermined type (either A or B) is added and its behaviour in the serum is
observed – and this should
confirm the result of forward typing.
The Rh-factor test also follows a similar pattern. Antibodies to Rh are mixed with the blood
sample, and if there is a
clumping reaction, the blood is Rh-positive.
