Why Do People Have Blood Types?

I started wondering about blood types yesterday, so I decided to do some research to learn more. It occurred to me that it's really odd that we're all human beings, but we have different blood that can be totally incompatible. I wondered why. This article contains my findings.


The Initial Discovery of Blood Types

By the late 1800's, scientists had noticed that when they mixed blood from different sick people in test tubes, sometimes the red blood cells stuck together in clumps. However, since they only tested sick people's blood, the clumping was assumed to be related to the source patient's illness and no further inquiry was made at that time. 

In 1900, an Austrian doctor named Karl Landsteiner had the idea to mix together different healthy people's blood. Some of the combinations clumped, while others did not. The clumping is what makes incomparable blood transfusions so dangerous, because clotting throughout the body chokes the overall system to death. 

Over time, Landsteiner isolated three "types" of blood into what he originally called Type A, Type B, and Type C blood. Type C was later renamed Type O, for reasons we will discuss shortly. Type AB was also discovered a few years later. Landsteiner received the Nobel prize in 1930 for his pioneering work on understanding blood. 

The Coating Around the Red Blood Cell

The primary difference between the blood types is the sort of coating around the red blood cells. The coatings are also known as antigens. The antigens are like a signal key that tells the immune system that these blood cells belong where they are. Cells containing an unrecognized signal key are attacked as a foreign invader. This is fundamentally why non-compatabile blood is possible. Non-compatible blood has the wrong signal antigen on the red blood cells, which is interpreted by the recipient's body as a foreign invader to be attacked. 

To be more specific, blood Type A has a certain sugar that sits on the surface of its red blood cells. We'll call that the "A" sugar. Blood Type B has a mutated form of the sugar, which we will call the "B" sugar. People with Type B blood have this B sugar sitting on their red blood cells. Type O blood is the absence of the A sugar and the B sugar. Type AB blood has both the A sugar and the B sugar on the red blood cells. Again, the body's immune system uses the combination of the A and/or B sugar or their absence to leave cells with the correct configuration alone and attack cells with the wrong configuration. 

Why is Type O the Universal Donor?

Type O is the universal donor because it has neither the A antigen nor the B antigen and it's the A and/or the B which signals the body to freak out if the combination is wrong. So a Type A blood will accept Type O blood because there is no B sugar to initiate a bad reaction. A Type B blood will accept Type O blood because there is no A sugar to initiate a bad reaction. Type AB blood is similar in that it already has both the A and the B sugar, so the Type O blood doesn't have any additional foreign signal to cause a problem. 

Type O can be easily given to other blood types, but the cost is that it's harder to donate blood for a Type O person. Using the same thinking as before, Type A, Type B, and Type AB all contain one or both of the "foreign invader" signals, so a Type O blood type will respond negatively to all of these donors. Only another Type O donor is compatible because this blood will have neither the A antigen nor the B antigen which enable the rejection reaction. 

What is the Plus or Minus Part?

There are another set of antigens that are independent of the A and the B ones. These other antigens are called Rh (which is short for Rhesus). Rh is actually a collection of 45 different antigens, but you either have all of them (+) or none of them (-).

Similar to the discussion about Type O blood above, positive blood can accept either positive or negative blood, whereas negative blood can only accept other negative blood. That's why the true universal donor is Type O-, which does not contain A, B, nor Rh. Conversely, AB+ can take any blood because it already contains A, B, and Rh. 

Are There Any Exceptions?

Yes. There is a tiny percentage of people who have neither A Type, B Type, nor Type O blood. These people are called the Bombay Phenotype because a few individuals were discovered with this type of blood in Bombay in 1952. It remains exceedingly rare. Those with the Bombay Phenotype cannot accept blood from anyone other than others with the Bombay Phenotype. Aside from the trouble obtaining compatible blood for a transfusion, there doesn't seem to be any negative health consequence from having blood in the Bombay Phenotype. 

In general, doctors now recognize 23 blood group systems with hundreds of different sub-types. However, the basic A, B, AB, O and + or - categorization functions well for the purpose of determining blood transfusion compatibility. This is still an evolving area of knowledge, so we would expect future discoveries to be made as more granularity in knowledge is obtained about the different ways blood can exist. 

What is the Evolutionary Basis for Blood Types?

The evolutionary basis for blood types seems to be disease resistance. Some types better defend against certain diseases and certain types against others. Malaria resistance seems to be encouraged in Type O blood, for example. Therefore, finding Type O blood is more common in areas of the world where Malaria is commonplace. 

Do Other Animals Have Blood Types?

Yes. Scientists are tracing blood types in other related animals to better understand when these mutations originally occurred in our lineage. Chimpanzees, which are our closest living relatives, only have Type A or Type O blood. Conversely, Gorillas only have Type B blood. The implications of these findings are still unclear at the present juncture.

What About Choosing My Diet Based on Blood Type?

There used to be a fad diet oriented around eating based on your blood type. The author of the book did not provide any scientific evidence for the validity of this approach and in general the scientific consensus seems to be that eating for your blood type in nonsense. However, some of the suggested eating patterns in the book are healthy for human beings in general, and therefore might result in better overall health, regardless of blood type. If you want to start eating in a more healthy manner, we suggest you check out our healthy eating page instead of trying a fad diet.

Does Blood Type Determine My Personality?

The short answer is no. I did locate a few claims about different personality templates between the different blood types, but the descriptions I saw were almost comically general. When reading the descriptions, I thought to myself that literally all of the descriptions were general enough to apply to me. Unless I see some compelling evidence otherwise, I'm going to continue to think that personality is primarily derived elsewhere. 

Concluding Thoughts

We hope this has been an interesting introduction about why human beings have different types of blood and what the implications are for having varied blood types. If you enjoyed this article, please consider sharing it on your social media. If you have a question or any sort of factual correction regarding this article, please contact us so that we can correct it. Thank you and have a wonderful day!