Not all human blood is exactly alike. each person has a particular combination of materials in his or her blood. This substances can react with each other when blood from two different people are mixed. If the wrong blood ifs given to a person, clumping of blood and death may occur. Safe transfusion of blood depend upon the careful matching of the blood of the people involved.
A scanning electron microscope image of normal circulating human blood. In addition to the irregularly shaped leukocytes, both red blood cells and many small disc-shaped platelets are visible. Source: Wikipedia.
There are many substances in blood but only few cause serious problems during transfusions. They are called A, B and O. A person whose blood has only substance A has type A blood. A person with only substance B has type B blood; one with both A and B has type AB blood; and one with neither A nor B has type O blood. All humans have one of these four types of blood.
A person who has type A also carries an anti-B substance in the blood. Since the B blood and anti-B substance react, a person with type A blood cannot receive a transfusion of type B blood. The table below illustrates the type of transfusion allowed for different blood types.
| Blood types | Blood substances | Best transfusion | Emergency transfusion |
|---|---|---|---|
| A | A, anti-B | A | O |
| B | B, anti-A | B | O |
| AB | A,B | AB | A, B, O |
| O | anti-A, anti-B | O | None |
The substances in a person’s blood are present at birth and can’t be changed by a normal transfusion. Type O and type A is most common in USA. Type B and type AB occur in less than 15% of the people. The actual percentage of each blood type vary among racial groups.
Note: A person may donate 450 ml of blood every two months.
