Human blood consists of red blood corpuscles (RBCs) as a constituent which gives it is red colour. On the surfaces of these red cells are present one or both of two types of antigens (proteins), designated as 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).
Antibodies have the property of clumping red cells. When antigen-A is present on the red cells, the serum contains only antibody-B that 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 that will clump red cells having antigen-A. As a result the blood is classified as group B.
In some people, both the antigens A and B are present on all the red cells and so their serum does not contain any of the antibodies. They belong to the AB group. As a result, their blood cells do not 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 prevent clumping of red cells, A group can get blood only from A and O, and B group only from B and O. But the AB group can get blood from any of the groups. Hence it is called universal recipient. On the other hand, an O group person can receive only O blood, but can donate blood to anybody else. Hence 0 is termed a universal donor this system of classification is known as 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 the other in which it is not (Rhesus negative).
