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Why we have different blood types and the advantages and disadvantages of each 


Phylogenetic studies suggest that blood type diversity is not neutral; rather, it’s maintained by selection (1). There are two major theories about why.
  1. Variation in selection. People with blood type AB are somewhat resistant to cholera, whilst type O are especially vulnerable to it (2). On the other hand, type O are resistant to malaria, whilst type B are at increased risk (3). So one can easily imagine that when cholera epidemics are frequent, A and B spread; when malaria strikes, O spreads. Since O is recessive, it’s unlikely to become extinct without strong selection, and O variants can arise rather easily because anything that renders the ABO gene non-functional is an O. Also, type O individuals also have a mildly increased risk of squamous and basal cell carcinoma, but decreased risk of pancreatic and gastric cancer. Type A is associated with increased risk of gastric cancer, and B is associated with increased risk of ovarian cancer.
  2. Negative frequency-dependent selection. This means individuals with rare blood types are advantaged. Why would this be? Firstly, because bacteria co-adapt to their hosts, so they are best able to attack the most common kinds of host. Secondly, because viruses steal cell membrane proteins from their hosts to disguise themselves from the immune system. So when a virus invades a new host, it’s likely to be carrying antigens from previous hosts. If these antigens are different from those in the new host, the immune system can recognise and attack the virus more easily (4). This mechanism may influence HIV-1 infection rates across blood types, for example (5).
SO, which explanation is correct? As you can see, there are plausible arguments for both, the evidence is rock-solid for neither, and both could be right at the same time. Isn’t science exciting?

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