Even though all ~blood~ is made of ~plasma~, red |blood cells|, white |blood cells|, and |platelets|, the ~blood~ of all people is not the same. There are four main blood types, or groups, into which all human ~blood~ can be placed. The discovery of these ~blood~ groups was made early in the twentieth century.
In identifying ~blood~ groups, only the red cells and the ~plasma~ are examined. ~Blood~ of certain types cannot be given safely to people with certain other types. When the red |blood cells| from one person are mixed with serum from another, there is sometimes, although not always a reaction. The raction, called aggutination, occurs because chemicals in the red corpuscles of one ~blood~ type may be incompatible with the chemicals in the ~plasma~ of the other type. If a reaction occurs, the red cells clump together. Serum is the clear liquid which is left when the ~blood~ clots. The serum is ~plasma~ without the chemicals which cause clotting. ~Blood~ groups are identified by checking the reaction of ~blood~ with known types of ~plasma~ or serum.
Every ~blood~ cell has a thin shell. The chemicals in this shell vary from person to person. Two of the most important of them are known as the A and B antigens. The absence or presence of these antigens is inherited. People who have the A antigen belong to ~blood~ group A; those with just the B antigen belong to group B. Most people have ~blood~ that belongs to one of the four main ~blood~ groups: A, B, AB, or O. A small number of people have both A and B antigens and belong in group AB. A larger portion of people have neither A nor B antigens and are classified as group O. All people may give or receive ~blood~ from their own group. People with type-O ~blood~ can give to any group. They are known as universal donors. The red cells in type-O ~blood~ will not be clumped by any other |blood plasma|. However, type-O people may receive ~blood~ only from other type-O donors. People with type-AB ~blood~ can receive ~blood~ from any of the four groups. ~Plasma~ in AB ~blood~ does not contain chemicals that will clot with any other red cells. Group AB ~blood~ is called the universal recipient. However, type-AB ~blood~ may not be given to any group other than itself.
The ABO system, as it is called, was discovered by Karl Landsteiner between 1900 and 1902. It is vital in transfusions. Before this discovery, hundreds of women died each year from hemorrage (loss of ~blood~) in |childbirth|. Because of transfusions, such deaths have now been greatly reduced.
