Thursday, 14 April 2011

Blood groups and bruises

Fig 1: I has a bruise.
I went to give blood last Thursday. Giving blood is something I feel quite strongly about, so I try to go as often as I can. Sadly I hadn't been in about a year - but I persuaded a friend to come and donate, too, so I felt less guilty! Unfortunately this time, for me, it didn't go too smoothly. I didn't actually manage to give blood, because the needle went in wrong. Hence the bruise. So I'm going back in a couple of weeks.

After you first give blood, you are sent a donor card, which tells you your blood type. I'm O+ - this was a bit of a surprise to me, as both my parents are A+! A quick Google cleared up why: the A blood group is inherited dominantly, so it's possible for someone to carry one copy of a gene that produces the A antigen, and one copy that doesn't, and still have the A blood group (they are heterozygous). So, both of my parents must carry an A allele and an O allele. (Blood types have actually been used to clear up paternity disputes, before more detailed genetic information was easily available.)

But what do these A, O (and B) things actually mean?
They refer to the A and B antigens: proteins that are found on the surface of red blood cells (O is not an antigen itself, it refers to the absence of the A and B antigens and so is sometimes written as 0 instead). These are produced from the same starting protein by an enzyme that modifies that protein, and which version of the enzyme you have determines whether it produces A or B. It's also possible to have a non-functional copy of the enzyme, which doesn't modify the starting protein. Each of my parents carries one working, A-producing copy, and one broken copy; I have two broken copies. Someone with an AB blood type carries one copy of an A-producing enzyme, and one copy of the B-producing enzyme, and their red blood cells will have both sorts of proteins. This is called codominance.

Fig 2: inheritance of ABO blood types, from Wikipedia
The + / - refers to the Rhesus D antigen, which is unrelated and inherited separately and in a more complicated way. About 85% of the population have the RhD antigen on their red blood cells, and are Rh+.

The 'starting protein' I mentioned above is confusingly not called the 'O antigen'. It's called the H antigen instead. The vast majority of people produce this starting protein, whether they have the A or B producing enzyme or neither. About 4 people out of every million, however, don't. They cannot produce the H antigen, and so cannot produce A or B either. This is called the 'hh phenotype', more commonly known as the Bombay phenotype. It's incredibly rare, except for in the area around Mumbai, where it's only very rare. The usual tests cannot show the difference between this and type O blood.

This is important, because the compatibility of blood groups is vital to the success of blood transfusions. Get it wrong, and people die. Karl Landsteiner, an Austrian doctor, won a Nobel prize for his work on identifying blood groups in humans, because it allowed (relatively) safe blood transfusions to take place for the first time.

We produce antibodies to the antigens that we do not produce ourselves. So, my blood contains anti-A and anti-B antibodies, and the blood of an AB- person would carry antibodies to the RhD antigen, but not to A or B. Antibodies are supposed to protect us; they recognise things that are not 'self', like viruses or bacteria, and attack them. In this case, they could also attack red blood cells from another blood type. Amazingly, the results of this can be seen by eye! In the image below, test blood has been mixed with three solutions: from left to right, these contain anti-B, anti-A and anti-RhD antibodies. See how the red blood cells seem to clump together in the middle spot, and less obviously in the right hand spot as well? If that happened inside you, it would be very bad indeed.

Fig 3: the slide test for blood groups, from Wikimedia Commons.
This blood sample has reacted with the anti-A and anti-RhD antibodies, so it's from someone who has both those antigens, i.e. is A+.

Okay, so knowing your blood group is important if you're going to give blood or receive a transfusion, but does it have any everyday relevance? Some people think so. In Japan, it is common to associate blood types with certain personality traits, for example type Bs are supposedly disorganised, while type Os are decisive and confident (ummm...). One man has written a book suggesting that your blood type should influence your diet! He even cites scientific papers, although the general scientific consensus is that there is no evidence for blood group having any influence on your dietary requirements, or your personality.

Do you know your blood type? Do you think you fit in with the Japanese stereotype for your group? More importantly - would you consider giving blood?


Nobel Lectures, Physiology or Medicine 1922-1941, Elsevier Publishing Company, Amsterdam, 1965 (here)


  1. I know my blood group (A rhesus negative) because I went to try and give blood once. A few minutes in to donating I completely passed out on the chair. I turns out my body doesn't *like* the idea of loosing blood, and tends to panic rather badly when it happens.

    The nurse told me to come back in about ten years time when my body was slightly less wussy about it :p

    1. I'm A negative and passed out giving blood once! crazy..

  2. @LabRat: Ah, that's not good! I suppose it makes sense for the body to not like losing blood though :P I've never fainted after giving blood, although the friend I persuaded to come along with me has - thankfully she was fine this time!

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  3. I've never given blood (I'm not eligible because apparently I'm underweight), but I know my blood type (O-) from being genotyped! It never occurred to me that I could find out that way, but just like everything else it has to be coded in my genes somewhere.

  4. I think the weight thing is mostly for your own protection - they could cause you to become anaemic, I think, and you'd be more likely to faint. Hell, I'm more likely to faint simply for being young and reasonably small, even though I'm definitely above the weight restriction. It's a shame though, because you have the blood type that's most useful!

    I wonder if you can find out your blood type under the other blood typing systems from being genotyped, too? I suppose it depends which SNPs are used...

  5. 23andMe tests SNPs for Diego, Kell, and Kidd blood types, but I have no idea what those mean.

  6. What I don't understand is how two RH- parents can produce AB+ children? Even after reading the above, I am still confused. I was told by a Doctor years ago that the children get the blood lineage from the father, now I see it is much more complicated than that.

  7. Anon - As far as I know, Rh- parents can't produce Rh+ children. It may be possible due to mutation in the child, but very unlikely if so and I'm not sure if this is even feasible, it depends on the exact change in the Rh- parents to start with. Was this something that was unclear in my post?

    As for children inheriting their blood type from their father, this can happen, but it is a bit more complicated. Rh- is the complete absence of the Rhesus D antigen. We all have two copies of each gene, so to be Rh-, you need two missing/broken copies. But to be Rh+, you only need one working copy - the other copy can be working or broken. Say the mother of a child is Rh-, and has two missing copies, and the father is Rh+ and has two working copies: the child inherits one of each sort - but you only need one working copy to be Rh+, so the child will have the same blood type as its father. Does that make sense? Hmm, a diagram might help...

    If both parents carry one of each sort, in theory the child could inherit both missing/broken copies and be Rh-, although both parents would be Rh+.

    A Rh- mother having a Rh+ baby can cause problems, actually. The mother's body hasn't seen the RhD antigen, which is suddenly produced by her baby, so this can lead her immune system to attack the baby's red blood cells and lead to anaemia, or "haemolytic disease of the new born". That happened with my dad, actually!

    Michelle - I've heard of the Kell antigen... Not sure what it does though. I think it was on House! :)

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