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Blood Groups Question Why have humans developed different blood types and is there an evolutionary advantage? Surely, if blood types are due to random mutations, one will be a better performer in general terms than the others? And why can't different types be used for all patients requiring transfusions? Answers There are four blood types: A, B, AB and O. These designations refer to the types of sugars (A, B and O) found on the surface of red blood cells. Everyone on the planet has an O sugar, and those who have no other type are known as blood group O. The other group names arise from the fact that some people have A, B or both A and B sugars attached to the O sugar. The genetics is unusual in that there are two equally dominant alleles. If the gene from your mother is the recessive allele for producing O cell surface markers, and the gene from your father is for A cell surface markers, then your overall blood type is A. Why? Since everyone has O we only look at the second sugar present to determine blood type. If the gene from your mother was for B and the gene from your father was A, then your blood type would be AB. If both parents donated A alleles, then you would be A. And if both gave you recessive alleles then your blood type would be O. Cells use things such as proteins and sugars on their surface for many purposes. One is to enable the immune system to tell "self" from "non-self" and distinguish "you" from every "foreign" body that may invade. Our cells have lots of different types of surface markers that tell the immune cells not only that they are self but also what type of cells they are. Red blood cells have the A, B and O markers and can also be rhesus positive or negative, depending on whether a separate marker is present or absent. Why can't any type of blood be given to anyone else? Our immune system attacks anything that isn't recognised as self. That means that if I have type A blood, meaning that I have both A and O sugars on my red blood cells, I can accept both type A and O blood from a donor. My body recognises both A and O as self. If type B blood was given by mistake, my immune system would attack those blood cells, and the transfusion would kill me. Type O blood is the universal donor, because nobody makes antibodies to this blood type--we all have the O sugar. Type AB blood is known as the universal acceptor because all blood sugars are recognised as self.
The ABO blood system provides us with an example of stable polymorphism (or "many forms") because the alleles responsible for the four different blood groups occur in fairly constant proportions. Alleles A and O are more common than B, so that about 40 per cent of people are in group A, 40 per cent in group O, fewer than 20 per cent in group B and only 2 per cent in group AB. These are global figures and there are some regional differences. For example Western Europe is dominated by groups A and O but Celts, such as the Irish, are nearly 80 per cent group B. The Indian subcontinent also has a preponderance of B alleles. At first sight these statistics are difficult to explain. Surely one blood group is as good as another? In fact, there is now strong evidence to indicate that blood groups confer protection or susceptibility to a wide range of human diseases. Groups A and AB are more susceptible to smallpox (thankfully eradicated), group A is associated with stomach cancer while group O has an increased likelihood of developing duodenal ulcers. The problem of blood groups and transfusions is related but different--it involves antibodies to a particular blood group. If a mistake is made in matching blood types, the patient's antibodies will treat the transfused blood as an invading infection and try to destroy the red blood cells it has received. Of course, this rarely happens unless a major mistake has occurred in medical operating procedures. A more common problem happens in pregnancy with the rhesus aspect of the blood system, when a rhesus negative mother carries a rhesus positive child. During the last month of pregnancy, fragments of fetal red blood cells containing the rhesus antigen cross the placental membrane into the mother's bloodstream; the mother responds by producing rhesus antibodies which later pass back to the fetus, destroying its red blood cells. This rarely does enough damage to affect a first child, but it sensitises the mother so that, if she conceives another rhesus positive child, her body will start producing antibodies much earlier in the pregnancy. This condition, known as erythroblastosis fetalis, is likely to kill the child unless it receives a blood transfusion of rhesus negative blood while it is still in the uterus. Prevention is now possible with an anti-rhesus globulin that coats the fetal cells and prevents the rhesus factor antigen entering the mother's blood in the first place.
Additionally, types resistant to diseases from one region may be at risk elsewhere. The distribution of blood groups reflects this. For example, antigens known as Duffy antigens are relatively rare in African populations; they seem to be associated with susceptibility to Plasmodium vivax malaria. Also A-type blood seems to go with susceptibility to the debilitating waterborne disease schistosomiasis (bilharzia), and this is consistent with the distribution of type A in Africa. Such distributions are a more common selective outcome in evolution than a quick takeover by just one allele. Here is a list of the blood types and their frequency in the UK population :
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