1.
With “Blood types” we mean what type of blood may or may not make blood transfusion possible.

2.
Long time ago, when blood transfusion become possible, it turned out that some people who received blood from another person were fine, while others suddenly died because their blood had started to coagulate!

3.
After a lot of work and research, it turns out that the surface of the erythrocytes contains an antigen.

4.
In fact, we now know that there are hundred possible types of antigens. The vast majority of them are harmless.

5.
But there are two types of antigen systems that turn out to be very crucial:

6.
These two systems are:
a. the ABO system
b. the Rhesus system

1.
Two types of antigens – type A and/or type B – may (or may not) occur on the surface of the erythrocytes.

2.
The genes on the chromosomes of that person determine the presence or absence of these two antigens.

3.
From your school days, you may remember that your DNA is stored as a set of chromosomes, actually a set of 23 pairs. This is your genotype.

4.
On chromosome 9, of which you have two, you could have antigen A, antigen B or no antigen at all (this is then called O).

5.
As you can see in this table, this means that the following combinations are possible in every person:

6.
Antigens O+O = Blood type O
Antigens A+O = Blood type A
Antigens A+A = Blood type A
Antigens B+O = Blood type B
Antigens B+B = Blood type B
Antigens A+B = Blood type AB

7.
All these combinations are called phenotypes. Every person has one specific phenotype. But not all phenotypes are equally distributed in the population.

8.
Some types are more common than others:
Blood type O – 47%
Blood type A – 41%
Blood type B – 9%
Blood type AB – 3%

1.
So, when you are born, you are born with your blood genes: A, B, AB or O.

2.
Obviously, your immune system will not develop antibodies to the type of gene you have in your chromosomes.

3.
But your body will get exposed to the other antigens that you do not have in your chromosomes. These antigens can be located in the food that you eat, or drinks, or bacteria etc.

4.
Your immune system will detect these ‘foreign’ antigens (as they should) and develop antibodies against these foreigners. This happens mostly in the first year of your life.

5.
So, after the first year, you will have developed antibodies against the antigens that you do not have in the chromosomes.

6.
As you can see in the table, someone with blood type A will therefore develop antibodies against blood type B. These are called anti-B agglutinins.

7.
Someone with blood type B will develop antibodies against antigen A: anti-agglutinin A.

8.
What happens to someone who has no antigens (= blood type O)? He/she will develop anti-agglutinin-A and anti-agglutinin-B!

9.
And someone who has blood type AB? That person will not develop any anti-agglutinin at all.

1.
So, before you give a blood transfusion to somebody, you first need to ‘match’ the blood type of the patient with that of the donor blood.

2.
You do this by taking a small amount of the blood, separate the erythrocytes from the plasma and mix the erythrocytes in one portion with serum containing anti-A agglutinins and a second portion with serum containing anti-B agglutinin.

3.
After a few minutes, you can see, under the microscope, if the erythrocytes are starting to stick to each other – agglutination. This means that there is a reaction of the erythrocytes to the agglutinin (=antibodies).

4.
So, if the blood is of the A type, it will agglutinate with anti-A but not with anti-B. A blood of type O will not show agglutination in either samples, while blood type AB will show agglutination in both samples.

5.
So, this is how you determine the blood type, either your own or that of the donor.

1.
In the Rhesus-system there are also many antigens in play, on the surface of the erythrocytes.

2.
But there is actually only one which is very common and plays an important role; the antigen D.

3.
The vast majority of people (85%) have this D antigen and these people are called Rhesus positive (= Rh+).

4.
But some 15% do not have this D-antigen and these are called Rhesus-negative (= Rh-).

5.
So, you now have to consider two systems when considering a blood transfusion; the ABO-system and the Rhesus system.

6.
This is usually abbreviated as A+ (= blood type A and Rhesus-positive), or AB- (=blood type AB and Rhesus negative), and so on.

7.
By the way, the Rhesus system is called ‘Rhesus’ because it was first discovered by Landsteiner and Weiner, in rhesus monkeys.

1.
The rhesus-system turns out to play an important role during pregnancies!

2.
In the vast majority of pregnancies, this is usually not a problem except for the following situation:

3.
This is the case when:
a) the father is Rh+
and b) the mother is Rh-
and c) the fetus is Rh+

4.
If this is the first child for this Rh- mother, no problem. The mother has not (yet) developed antibodies against the Rhesus antigen.

5.
But at the end of pregnancy, and especially during delivery, some of the Rh+ erythrocytes of the baby may enter the blood circulation of the Rh- mother.

6.
This will induce the immune system of the mother to form antibodies against the Rhesus factor. This is not a problem for the mother as she is Rh- anyway.

7.
But if she gets pregnant a second time, and the fetus is once more Rh+, then the fetus can be attacked by the Rh-antibodies from the mother. These antibodies can flow through the placenta into the circulation of the fetus!

8.
Fortunately, if one is aware of the Rhesus-status of the mother, one can avoid the formation of Rhesus-antibodies by injecting a Rhesus-vaccine (an immunoglobulin) after the delivery. These will destroy any Rhesus-positive blood cells from the fetus that have sneaked into the mother’s circulation.