Human red cells have a variety of antigens on their surface. The first
system used for these antigens was the ABO system. ABO antigen status
is inherited, it consists of three allelic genes, A, B and O. Both the
A and the B genes code for the transferase enzymes. These convert a simple
antigen, H, found on every red cell membrane into either antigen A or
antigen B. The O gene has no effect on the H antigen, it is amorphous.
Six different genotypes exist that give rise to four different phenotypes.
| Phenotype | Genotype | Antigen on red cell | Antibody in serum |
| A | AA or AO | A | anti-B |
| B | BB or BO | B | anti-A |
| AB | AB | A and B | none |
| O | OO | none | anti-A and anti-B |
The Rhesus system consists of the gene RhD that encodes the D antigen.
Absence of the D antigen is denoted 'd'. Individuals possessing this D
antigen are termed Rhesus positive and those lacking the antigen are Rhesus
negative.
|
Phenotype
|
Genotype
|
|
Rh
+ve
|
DD
or Dd
|
|
Rh
-ve
|
DD
|
Antibody anti-D is only generated after a Rhesus negative individual is
exposed to Rhesus positive red cells.
The percentage of each type of blood type varies between countries. Blood
types A and O are generally the most common but in certain parts of Africa,
B is usually the most common type. Rhesus positive is normally much more
common than Rhesus negative.
|
Area
|
ABO
System
|
Rhesus
System
|
||||
|
O
|
A
|
B
|
AB
|
+ve
|
-ve
|
|
|
Europe
|
46%
|
44%
|
7%
|
3%
|
83%
|
17%
|
|
West Africa
|
9%
|
5%
|
84%
|
2%
|
97%
|
3%
|
|
North India
|
38%
|
40%
|
19%
|
3%
|
90%
|
10%
|
|
China
|
38%
|
31%
|
29%
|
2%
|
93%
|
7%
|
|
Japan
|
-
|
-
|
-
|
-
|
99.7%
|
0.3%
|
When blood transfusions are carried out the blood types of the recipient
and donor are very important. It has to be made sure that certain antigens
and antibodies are not mixed. Each blood type can only have a certain
blood donor.
|
Recipient
|
Donor
|
|
A
|
A
and O
|
|
B
|
B
and O
|
|
AB
|
A,
B, AB and O
|
|
O
|
O
|
Blood type AB is the universal recipient, it does not possess anti-A
or anti-B antibodies and so can accept all blood types. Blood type O is
the universal donor, it is not affected by the antibodies anti-A
or Anti-B and so can be given to all blood types.
If you are interested in donating blood please visit the National
Blood Service website for more information.
Enter both
parents blood types and find out all the possible blood types of their
children or enter the blood type of the child and find out the possible
combinations of blood types of the parents.
Test yourself on the ABO system. How much do you know about how blood types are inherited? Find out by taking the quiz. See where you come on the scoreboard, can you come top?