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There are four types of blood among the humans. These are called the blood groups. The blood groups are classified based on the presence or absence of certain proteins and antibodies.
In human blood, the membranes of the Red blood Corpuscle have certain proteins called the antigens. There are two types of antigens:
- A antigen
- B antigen
The blood that has antigen A belongs to group A, the blood that has antigen B belongs to group B, the blood that has both A and B belongs to group AB and the blood which has neither belongs to group O. The presence of these antigens is determined genetically. Thus, there are four types of blood groups A, B, AB and O. This classification was done by Karl Landsteiner in 1900. Most people have O group (46%), followed by A group (42%), then by B group (9%) and least number of people have the AB group (3%)
The blood also contains antibodies which act against these antigens. The antibodies react to the presence of proteins in the foreign bodies. There are two types of antibodies, corresponding to the types of antigens.
The two antibodies are:
- antibody a
- antibody b
Anti-body a reacts to the antigen A and antibody b reacts to the antigen b. Thus, the blood of A group that contains antigen A will contain antibody b and not antibody a. The blood of B group that contains antigen B will contain antibody a and not antibody b. The blood of AB group that contains antigens A and B will not contain either antibody a or antibody b. The blood of O group that does not contain either antigen A or antigen B will contain both antibody a and antibody b.
| Blood Group | O | A | B | AB |
|---|---|---|---|---|
| Percentage of population | 46% | 42% | 9% | 3% |
| Antigen | - | A | B | A + B |
| Antibody | a + b | b | a | - |
These antibodies are present in blood always and not produced in reaction to the transfusion. The reaction between the antigen and antibody results in the Red blood Corpuscles sticking together. This is called coagulation or agglutination. The antigens are also called agglutinogens and antibodies are also called agglutinins.
| Blood Group | Antigen Present in RBC | Antibody Present in Plasma | Possibility of Blood Donation |
|---|---|---|---|
| A | A | b | Can donate to A and B |
| B | B | a | Can donate to B and AB |
| AB | A, B | none | Can donate from all but donate to only AB |
| O | none | a, b | Can donate to all but receive blood only from O |
Rhesus Factor
In addition to the antigens, there is another agglutinogen that is present in almost 85% of the people. This factor was present in the blood of the Rhesus monkeys and is called the Rhesus factor. The blood that contains this factor is called Rh+ve and the blood that does not contain this factor is called Rh-ve.Rh-ve
The Rh-ve blood does not contain any anti-Rh agglutinins initially. They are produced in the blood when the Rh+ve blood is mixed with the Rh-ve blood.Rh-ve blood can be safely given to Rh+ve person. However, the transfusion of Rh+ve blood to Rh-ve blood needs to be monitored. During the first transfusion, the Rh factor in the Rh+ve blood will induce the production of anti-Rh antibodies in the Rh-ve blood. These will cause agglutination during any further transfusion from the Rh+ve blood.
The Rh factor can cause clotting in the blood of Rh+ve foetus in the womb of an Rh-ve mother. The first Rh+ve foetus induces the formation of antibodies in the blood of the mother. These antibodies remain in the blood of the mother. If the second foetus is also Rh+ve the antibodies can harm the foetus if they enter its blood.