|Fig 1: I has a bruise.|
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 '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)