BLOOD GROUPS & BLOOD CLOTTING [CSEC BIOLOGY & HSB]
SYLLABUS OBJECTIVES
Red blood cells have proteins on their surface called antigens.
The blood plasma has another set of proteins.
Antibodies attack foreign antibodies.
There are two antigens, producing the following blood groups based on their combinations:
- Blood group A
- Blood group B
- Blood group AB
- Blood group O
For every antigen on the red blood cell, the opposite antibody will be in the plasma.
Transfusions
If blood of an incompatible group is transfused, the cells of the donated blood clumps. The clumps of rbc's block capillaries & other small blood vessels, and the patient experiences severe pain. The clumped rbc's are haemolysed, releasing high concentrations of haemoglobin into the plasma. This can lead to kidney failure and eventual death.
Because of this human blood transfusions before 1900 often had fatal results. Then Landsteiner introduced the concept of blood groups, which has been used to guide which types of blood can be mixed since then.
According to this blood group system, compatible blood groups are those that do not have matching antigens and antibodies.
For example, blood group A has anti-B antibodies. So therefore it cannot be mixed with blood group B, which has B antigens. However, blood group A can, of course, be mixed with blood group A.
Note that it is the donated blood cells that clump. Therefore, it is the antibodies in the recipient's plasma that attacks the antigens in the donor's cells. Hence, this is why:
- A person with blood group AB can receive blood from any other group, because there are no antibodies in its plasma. it is thus called a universal recipient.
- Blood group O has no antigens, so it can be given to most other people. It is therefore called the universal donor.
The rules of group compatibility with respect to transfusion can be shown thus:
Why the Focus on the Donated Blood?
With a limited transfusion, the effect of the donor's plasma on the recipient's rbc's may be ignored as the dilution of the donor's plasma by the recipient's plasma will lower the antibody concentration to the point of being inactive.
This was discovered in 1940. Unlike the ABO system there are no preformed antibodies present. One can either have the factor present (Rh D+) or absent (Rh D-).
Rh+ persons cannot produce anti-D, so they can receive blood from both groups, with no harm.
Only (-) persons can make the antibody, called anti-D, and only after being exposed to (+) blood.
Thus if a (-) person gets a transfusion with (+) blood, no external signs or symptoms of mismatch is apparent. The production of anti-D occurs slowly, and accumulates in the patient's body. However, any future reception of (+) leads to clumping and haemolysis of red blood cells.
Pregnancy
If a woman is Rh D- and her partner is Rh D+, their child could be either (+) and (-).
Rh+ is the dominant form and Rh- is the recessive form. Therefore, one is more likely to have an Rh+ baby if one partner is (-) and the other is (+).
If a child is (-), there is no problem unless the mother previously got transfusion of (+) blood.
If a child is (+), there is a risk that her blood will be contaminated with the baby's blood while giving birth. That first child would not be affected. However, her body will then develop & accumulate antibodies against the Rhesus blood group, called anti-D antibodies.
In a future pregnancy, if the foetus is also (+), those anti-D antibodies will cross the placenta and destroy the foetus' blood cells, which may lead to its death.
Prevention
To prevent this, Rh- mothers with a first Rh+ baby are given an injection of anti-D antibodies to neutralise any (+) antigens from fetus. This prevents any further anti-D antibodies from being synthesized by the mother's immune system. This protects any future pregnancies.
