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Blood types are not just a simple letter. While it’s not something we tend to think about every day (except if you’re a hematologist, phlebotomist, or perhaps a vampire), blood types have been tied to history, health, and, supposedly, personality. It’s more important than you may even imagine.
What are blood types? Most people have a certain letter blood type with a positive or negative attached to it (we’ll get to those special exceptions later). The standard letters are A, B, AB, and O, and they signify what kinds of antigens are on the cells (antigens are substances that give a certain response when a foreign substance comes into the body) (American Red Cross). There are A antigens and B antigens. A and B blood cells have their letter antigens respectively, AB blood cells have both, and O blood cells don’t have any of those antigens at all. The positive and negative signs signify another set of antigens called the Rh factor (American Pregnancy Association). Cells that have a key component of the Rh antigen set are given a positive sign, and those that don’t have that component are denoted with negative sign.
When it comes to donating blood cells, people with an A blood type can only provide blood for those with A blood and AB blood, and B blood type people are the reverse, where they can only provide blood to those with B blood and AB blood. AB blood can only be used for AB blood type people. O blood types are extremely special. Anyone can take in O blood, no matter what type of blood they have. However, people with O blood types can only take in O blood. People with positive blood can only give their blood to other people with a positive blood type, and negative blood type people can give blood to both positive and negative blood types. People with O negative blood are considered universal donors for this reason, since any person can take their blood without rejecting the transfusion. However, people with O negative blood can only have O negative blood. On the other hand, those with AB positive blood are considered universal receivers, since they can take any kind of blood. If someone receives the wrong type of blood, their body starts to undergo an ABO incompatibility reaction, which is extremely dangerous and, in some cases, fatal (Healthline). If the body receives the wrong type of blood, it will reject it, causing the red blood cells to break down and clot. Unless treated right away, a person’s blood can clot, causing a stroke, or the patient can undergo kidney failure because of all the dying cells.
Some blood types are more common than others. For instance, the rarest standard blood type is AB-, which accounts for one percent of the population (Disabled World). B- and O- blood are the second rarest, accounting for less than 5% of the population each. People with O- blood are commonly sought after because of their rarity and the fact that they’re universal donors. People with these blood types will often put their own blood in a bank if they know that they are having surgery that may require a transfusion. Some blood types are even rarer, and some nationalities have extremely specific blood types. For instance, there is a blood type in India referred to as hh, or the Bombay Blood Type (The Hindi). Those with this blood type can only accept blood form the same blood type, and the blood type exists in only 1 out of 10,000 Indians. There is also a possibility that a person’s blood is neither positive nor negative. If someone’s blood doesn’t have any of the Rh antigens, it is referred to as Rh-null (Disabled World). The difference between O and B negative and Rh-null is that negative blood is missing a key component in the Rh antigen set, but those with Rh-null blood don't have any of the antigens at all. Rh-null blood is extremely rare; there are only 9 active donors in the world with this blood. They are extremely important, since they can act as universal donors for anyone with a rare blood Rh blood type. However, that also means that they can only take the blood of other people with Rh-null blood.
Why do we even have these blood types in the first place? It seems pretty impractical and confusing if some people can take some kinds of blood, but others can’t, and messing the blood types up can lead to deadly consequences. Scientists look to genetics for an answer (Scientific American). Scientists theorize that the spread of blood types has to do with environmental pressures and evolution. For instance, there is a blood type called the Duffy blood type where those with a positive Duffy type are more susceptible to malaria. In parts of Africa where malaria is common, the Duffy positive blood type is much rarer. Scientists are still trying to find out what the purpose of A, B, and O blood are. There are certain statistical patterns with health conditions, such as A blood type people having a much higher likelihood of developing stomach cancer. Some blood types are also more susceptible to infectious diseases. For instance, people with O blood are more susceptible to the bubonic plague, and those with an A blood type or more susceptible to smallpox. Scientists theorize that this is the reason why people with the B blood type are so common in places like Russia and India where there were both of these plagues.
Blood types have captured the fascination of people, including scientists. In Japan, there is even a theory about one’s personality based on one’s blood type (PsycholoGenie). The tradition started in the seventies when a Japanese journalist named Mashiko Nomi discovered a scientific paper from 1927 trying to correlate personality traits with blood types. While the correlation has been denied by scientists, many of the magazine readers took to it, and the theory is still commonly applied today. For instance, those with A blood are loyal and sensitive, but they are also obsessive and stubborn. On the other hand, those with B blood are creative and hardworking, but they also are forgetful and insensitive. The relationship between connecting blood type and personality is akin to connecting one’s behavior to their zodiac sign. There is pushback over the validity of these personality claims, but these theories are extremely popular nevertheless.
Our blood type plays a larger role than one may think. Our blood type tells a biological history of evolution that we have yet to totally figure out. In medical science, knowing a patient’s blood type is extremely important, as giving a patient the wrong blood can lead to dire consequences. We attach personalities to blood types like we do to birthdays. It will be interesting to see what new discoveries will be made regarding our history and the secrets our blood type holds.