Blood Types

Chat to Melba about Blood Types, and the Rh factor (also known as "D" Antigens)

First a Definition: Antigen is a substance that causes production of (generates) an antibody. First used 1908 in German, from French antigène (1899)

Extract from www.science.org.au

The presence or absence of A or B antigens gives us four main blood types:


Thus, antigens are molecules capable of stimulating an immune response.
Each antigen has distinct surface features, or epitopes, sites on the surface of an antigen to which an antibody attaches itself, resulting in specific responses.

Antibodies (immunoglobins) are Y-shaped proteins produced by B cells of the immune system in response to exposure to antigens. Each antibody contains a paratope, that part of the molecule of an antibody that binds itself to an epitope. So it recognizes a specific epitope on an antigen and acts like a lock and key binding mechanism. This binding helps to eliminate antigens from the body, either by direct neutralization or by ‘tagging’ for other arms of the immune system.

So, the flipside of having these particular antigens is the production of particular antibodies in your blood’s plasma. These are produced in the first few years of your life in response to food, bacteria and viruses which you encounter. Antibodies are specialised immune proteins that are produced based on the antigens that are not present on your red blood cells. For example, if you have A antigens, you will develop only anti-B antibodies. So:


All this means that it’s really important that, if you happen to need a blood transfusion, you get the right type. That’s because if an antigen is introduced into your body which it doesn’t already have, rather than welcoming it with open arms, your system will identify it as an intruder. Alarm bells will start ringing (figuratively speaking), your immune system will go into attack mode, and antibodies will be produced to fight off the unfamiliar visitors.  

Say, my stepdad, who has O-type blood, needed a blood transfusion. What would happen if he received my B type blood? Having O type blood, remember, means that he has no A or B antigens on the surface of his red blood cells. If my cells, with their B antigens, were introduced into his body, his immune system would identify them as foreign, producing antibodies to provide immunity against them. 

Antibodies attack by binding to the foreign antigens, making the red blood cells clump together. If only small amounts of my blood were introduced into my stepdad’s system, this wouldn’t necessarily be a problem, as the rejected blood would be filtered out by his kidneys. But if he needed a larger-scale transfusion, his kidneys might not be able to cope. The result could be kidney failure and, potentially, death. 

What about if the situation is reversed? Could my stepdad donate his blood to me? Because his O type blood doesn’t have any A or B antigens, it could be safely introduced into my body—it has no unfamiliar antigens for my system to identify as intruders. In fact, because it has no A or B antigens at all, O (negative) blood can be donated to anyone, regardless of blood type. This is why it’s known as the ‘universal donor’.

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The "Rh factor" Positive or Negative, known as the D antigen

Extract from www.nhs.uk

The D antigen is a protein with many parts, found on the surface of red blood cells. The D antigen is also known as the "Rh factor," and it tells your blood type. People are either D (Rh) negative or D (Rh) positive. "Partial D" means that one or more parts of the D antigen are missing.


Rhesus disease, what causes it ?

Rhesus disease only happens when 1. the mother has rhesus negative blood (RhD negative) and 2. the baby in her womb has rhesus positive blood (RhD positive) and 3. the mother must have also been previously sensitised to RhD positive blood.

Sensitisation happens when a woman with RhD negative blood is exposed to RhD positive blood, usually during a previous pregnancy with an RhD positive baby. The woman’s body responds to the RhD positive blood by producing antibodies (infection-fighting molecules) that recognise the foreign blood cells and destroy them.

If sensitisation occurs, the next time the woman is exposed to RhD positive blood, her body produces antibodies immediately. If she's pregnant with an RhD positive baby, the antibodies can cross the placenta, causing rhesus disease in the unborn baby. The antibodies can continue attacking the baby's red blood cells for a few months after birth.



Preventing rhesus disease

Rhesus disease is uncommon these days because it can usually be prevented using injections of a medication called anti-D immunoglobulin.

All women are offered blood tests as part of their antenatal screening to determine whether their blood is RhD negative or positive. If the mother is RhD negative, she'll be offered injections of anti-D immunoglobulin at certain points in her pregnancy when she may be exposed to the baby's red blood cells. This anti-D immunoglobulin helps to remove the RhD foetal blood cells before they can cause sensitisation.

If a woman has already developed anti-D antibodies from a previous pregnancy (she's already sensitised) then these immunoglobulin injections don't help. The pregnancy will be monitored more closely than usual, as will the baby after delivery.



Treating rhesus disease

If an unborn baby does develop rhesus disease, treatment depends on how severe it is. A blood transfusion to the unborn baby may be needed in more severe cases. After delivery, the child is likely to be admitted to a neonatal unit (a hospital unit that specialises in caring for newborn babies).

Treatment for rhesus disease after delivery can include a light treatment called phototherapy, blood transfusions, and an injection of a solution of antibodies (intravenous immunoglobulin) to prevent red blood cells being destroyed.

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