A: Human blood consists of red blood corpuscles as a constituent, which gives it its red colour. On the surfaces of these red cells are present one or both of two types of antigens (proteins), designated A and B. Other than these, two antibodies, designated as antibody-A and antibody-B, present in the serum, are also involved in the classification of human blood. (Serum, a constituent of blood, is a straw-coloured liquid that can be seen after removing all the other blood cells from a sample.)
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Antibodies have the property of clumping red cells. When antigen-A is present on the red cells, the serum contains only antibody-B, which will clump red cells having antigen-B on their surface. Then the blood is classified as group A.
When antigen-B is present on the red cells, the serum contains only antibody-A, which clumps red cells having antigen-A. As a result the blood is classified as group B.
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In some people, both antigens A and B are present on all the red cells, so their serum does not contain any of the antibodies. They belong to the AB group. Their blood cells don’t clump whether they receive A group or B group blood. That is, A and B are compatible with the AB group.
The fourth type, O, has neither of the antigens on its red cells but has both antibodies in the serum.
In order to keep red cells from clumping, those with A group blood can receive blood only A and O group blood, and those with B group only B and O group blood. But the AB group can receive blood from any of the groups. Thus it is called the universal recipient. Similarly, those with O group blood are universal donors.
This system of classification is called the ABO system. Blood groups are also classified by the Rhesus system (Rh). The-Rh factor leads to one type in which the Rh factor is present (Rhesus positive) and another in which it is not (Rhesus negative).