Table of Contents
Page created on October 20, 2019. Last updated on January 24, 2022 at 16:06
Cell antigens
All cells have many proteins on the outside of their cell membranes. We say that the cells express these proteins. Different cells express different proteins, which allows them to interact with other cells and the environment differently.
All proteins are also antigens. An antigen is a molecule which the immune system can recognize. When a person is infected with a bacterium for example, the immune cells will recognize the antigens (proteins) on the surface of the bacteria as foreign and mount an immune response against that bacterium.
A person’s immune system normally doesn’t recognize and bind to antigens found on the person’s own cells. The immune system is “trained” to be able to differentiate the person’s own antigens (so-called self-antigens) from foreign antigens.
The immune system recognizes foreign antigens with the use of antibodies. Antibodies are large proteins in the blood which bind to foreign antigens. If a foreign antigen is introduced into the body, antibodies will bind to the antigen, which signals to the immune system that there is an intruder.
There are multiple different classes of antibodies. The two which are important for this topic are IgG and IgM. IgG antibodies are relatively small and can cross the placenta. Anti-D antibodies are of the IgG type. On the other hand, IgM antibodies are very large and cannot cross the placenta. Anti-A and anti-B antibodies cannot cross the placenta.
Blood groups
Like all cells, RBCs express antigens. Which antigens your RBCs express determine which blood group you belong to. People in the same blood group express the same antigens on their RBCs. The most important blood group is the ABO blood group, which differentiates people into one of four blood types. A person is either A, B, AB or O.
There are many blood groups but only two are important, the ABO and the Rhesus blood groups.
The concept of blood groups is important for blood transfusions. If I need to get blood transfusions the blood, I receive must be the same blood type as I am. Otherwise, my immune system would react against and haemolyse (destroy) the red blood cells that I received.
ABO blood group
There are four blood types in the ABO blood group system. A person is either A, B, AB or O. What blood type a person is depends on whether the persons RBCs express A-antigen, B-antigen, both A and B-antigen or neither. The blood group is genetically determined, meaning that it is inherited.
The following table shows which blood group you are depending on which antigens your RBCs express.
|
Antigens on RBC surface |
Your blood type |
|
Only A-antigen |
A |
|
Only B-antigen |
B |
| Both A and B-antigen |
AB |
| Neither A nor B-antigen |
O |
Everyone has antibodies which recognize foreign antigens, but no one has antibodies which recognizes self-antigens. From this we can understand that a person whose RBCs express A-antigen must have antibodies which recognize the B-antigen, because for that person the B-antigen would be “foreign”. If we take this logic further, we can determine which antibodies each blood type has.
|
Antigens on RBC surface |
Your blood type | Antibodies in your blood |
|
Only A-antigen |
A | Antibodies against B-antigen |
|
Only B-antigen |
B |
Antibodies against A-antigen |
| Both A and B-antigen | AB |
No antibodies against A or B-antigen |
| Neither A nor B-antigen | O |
Antibodies against both A and B-antigen |
Antibodies against A-antigen are often called anti-A antibodies and antibodies against B-antigens are often called anti-B antibodies.
The person who discovered the ABO blood group system is called Karl Landsteiner. He came up with two “laws” that apply to the ABO blood group system:
- A person does not have antibodies against their own antigens
- Every person has antibodies against the antigens they lack
H-antigen:
When RBCs are developing, they express an antigen called H-antigen. The H-antigen is an intermediate in the synthesis of A-antigen and B-antigen. If the person these RBCs are developing in is blood type A the H-antigen will be converted into the A-antigen. If the person is blood type B the H-antigen will be converted into the B-antigen.
If the person is blood type O the H-antigen will not be converted into anything and will remain unchanged. As such, RBCs of type O people express H-antigen.
A very rare blood type called Bombay type is characterised by having RBCs that express neither A-antigen, B-antigen nor H-antigen.
Rhesus blood group system
The Rhesus blood group system, often called just the Rh blood group system, is simpler than the ABO system. Any person is either Rhesus positive (Rh+) or Rhesus negative (Rh-). If a person is type B and Rh+ we call that person type B+, etc.
People who are Rh+ express an antigen called D-antigen on their RBCs. People who are Rh- don’t express this antigen on their RBCs.
In the ABO system every person has antibodies against the antigens they themselves lack. This is not true for the Rhesus system. An Rh-negative person doesn’t have anti-D antibodies unless they’ve been exposed to Rh-positive blood before.
|
Blood group |
Antigen on surface of RBC | Antibody in plasma |
|
Rh positive |
D-antigen |
No anti-D antibody |
| Rh negative | No D-antigen |
Anti-D is present only if previously exposed to Rh+ blood. |
Erythroblastosis fetalis
Erythroblastosis foetalis, also called haemolytic disease of the new-born (HDN) is a condition that can occur in mothers who have their second pregnancy with an Rh+ foetus, if the first pregnancy was also with an Rh+ foetus.
An Rh- mother doesn’t have anti-D antibodies, so there is no problem when they have an Rh+ foetus for the first time. However, under labour the mother’s uterus will inevitably come in contact with some of the Rh+ foetus’ blood. This will cause the mother to form anti-D antibodies. These antibodies don’t do anything, yet. These antibodies are of the IgG type and can therefore cross the placenta during the next pregnancy.
If the same mother has a second pregnancy, and the foetus this time is also Rh+, problems occur. The anti-D antibodies in the mother will cross the placenta and enter the blood of the foetus, causing the foetal RBCs to haemolyse, often killing the foetus.
Erythroblastosis foetalis doesn’t occur when there is ABO incompatibility (if the mother is A and the baby is B for example) because the anti-A and anti-B antibodies are of IgM type and therefore can’t cross the placenta.
Blood transfusion
When a patient needs a blood transfusion it’s important to give the correct blood type so that incompatibility doesn’t occur. Incompatibility can cause jaundice, kidney damage or worse.
It’s not always necessary to give the same blood type as the patient. The only requirement in blood transfusion is that the patient doesn’t receive blood which contains antigens the patient has antibodies against.
A person with blood group A has anti-B antibodies. They can therefore receive blood of any type that does not contain B-antigen. As such a person with blood group A can receive type A and type O blood.
The following table shows which blood type each blood type can receive. For the ABO system:
|
Blood type |
Antigen on RBCs | Antibodies in plasma | Can receive blood from |
|
A |
A-antigen | Anti-B antibodies | A and O |
|
B |
B-antigen | Anti-A antibodies |
B and O |
| AB | A-antigen and B-antigen | No anti-A or anti-B antibodies |
A, B, AB and O |
| O | Neither A-antigen nor B-antigen | Anti-A and anti-B antibodies |
O |
And for the Rhesus system:
|
Blood type |
Antigen on RBCs | Antibodies in plasma | Can receive blood from |
|
Rhesus positive |
D-antigen |
No anti-D antibodies |
Rhesus positive, rhesus negative |
| Rhesus negative | No D-antigen | Anti-D is present only if previously exposed to Rh+ blood. |
Rhesus negative |
As such blood type O is called the “universal donor” because they can donate to all blood types, and blood type AB is called the “universal recipient” because they can receive from all blood types.