Why do we have different blood types?

As a kid when I felt down, my mother used to say “Cheer up kid, after all, you are a B +ve. And I used to think that all the positive people in the world had B+ve blood group and all the pessimists had B –ve. I stuck on to that idea for quite a while before realizing how silly it was. Later in my science classes, I learned the types of the proteins and sugars, inherited from our parents, which determine our blood group. These sugars found (or expressed) on the surface of red blood cells define blood types and is coded by a gene (I) which has three alleles – Ia, Ib and I. Humans are diploid organisms and posses any two these three alleles. Ia and Ib are completely dominant over I. When both Ia and Ib are present together both of them express their own kind of proteins.

Based on the sugar coat of our red blood cell we can all be classified into blood groups= A, B, AB or O. A blood type can either have two Ia alleles or an Ia and I allele. Similarly B can either be two Ib alleles or a Ib and I, O is two I alleles and AB has one of Ia and Ib alleles.

The sugar coats are what make some blood types incompatible for transfusion. If blood from a person with blood type A is transferred to B blood type person, the receipient’s immune system would recognize them as a foreign particle and starts attacking. And the resulting immune reaction can be fatal. This might provide you with a hint as to why O is the universal blood donor and AB is the universal acceptor. O does not have any sugar molecule that would cause an attack and hence no immune reaction is closed. AB has both Ia and Ib allele sugar and hence both the alleles are native to the recipient’s immune system.

We are further organized into positive and negative, based on the presence or absence of the protein Rhesus D (RhD) antigen. This gives us eight main blood types: O-, O+, B-, B+, A-, A+, AB-, and AB+. The most ancient blood group is thought to be type A. Then came O and B.

What is the evolutionary significance of having different blood types?

Photo Credit: InvictaHOG/Wikipedia

After almost a century, the Nobel Prize was awarded to Austrian physician Karl Landsteiner for the discovery of blood types, we still don’t know exactly what they’re for. Science has developed several tools for understanding the biology behind the blood types. They have identified the evolutionary history behind the blood types and the various influences of blood types on our health. The selective environmental pressures can be thought to be essential for the persistence of certain blood types. For example, the absence of Duffy antigen in the red blood cell offers a measure of protection against malaria. The Duffy blood type carries a receptor that makes people susceptible to certain malarial parasites. Thus in some regions of Africa where the malarial infection rate is high, populations with Duffy negative blood type would have a selective advantage.

The functions of A and B blood group factors remain unknown. In addition to the blood cells, these factors are expressed in many other cells and tissues. The demographic patterns of the blood types are astonishingly yet unexplained. Group O individuals appear to have a higher risk of developing ulcers and ruptured Achilles’ tendons whereas people with blood group A are more susceptible to the risk of several types of cancer and smallpox virus. Type O blood group people are better protected against malaria than other blood groups. Studies indicate that immune cells can find malarial parasite-infected cells much easier if the person is type O. Diseases might have played a role in the evolution of blood types. But research hasn’t yet shown whether that or some other disease explains why humans still have blood types.

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