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The human blood group classification is based on these antigens on our Red Blood Cells (RBC), and there are around 32 Human Blood Group classifications based on the various antigen groups.
ABO is a classification of human blood group based on the presence or absence of A or B antigens.
So if your blood has only A antigens, then your blood group is A, if it has only B antigens, then your blood group is B, if it has both of these, then your blood group is AB, and if it has neither, then your blood group is O. Now the reason for taking into account this group for blood transfusion is that, if you have A antigen (A Blood Group), then the human immune system will not attack A antigens if the donor’s blood contains A.
However if a person with B or AB blood group is given to a person whose blood group is A, then the recipient’s body will attack the B antigens in the received blood as it has not been trained to ignore B antigens.
Some evolutionary gene modifications might prove fatal when exposed, but are nevertheless are useful while they remain dormant and hence are continued to be carried.
For instance, Sickle Cell anemia is a fatal disease where RBCs sickle up causing life threatening complications.
This is also the reason why blood groups like O-ve are very rare.
Most humans have Rh D antigen, and are hence Rh positive. So what caused this mutation where some humans stopped having this antigen in their blood? It is interesting to note that Rh -ve or the absence of Rh D producing genes is common in the European population, even though it is rare in rest of the world.