Why Do Humans Have Different Blood Types?

Although all blood is made of the same basic elements, not all blood is the same. The difference in blood might seem like an unnecessary trait in a species, even more so when non-compatible blood is harmful or even fatal when injected into our body. To understand the significance of different blood types, we first need to understand the distinction.

Human blood is categorized into four different types: A, B, AB, and O. Before the 19th century when a patient lost too much blood, erroneous practices of animal blood transfusion cost many lives. Most scientists preferred avoiding the practice of transfusion, even from humans. Blood groups were first discovered by an Austrian Scientist named Karl Landsteiner in 1900, for which he also won the Nobel Prize.

Karl Landsteiner. Source- fineartamerica

The differentiation is done based on the presence of a certain type of protein on the surface of red blood cells in our body. This protein, or antigen, has two types, namely A and B. A person whose blood group is A has the antigen A, while a person whose blood group is B has the antigen B on the surface of their red blood cells. However, a person who has neither A nor B antigen has the blood group O, while a person who has both A and B has the blood group AB.MEME blood type

What are antigens?

Antigens are basically chemical markers of RBCs that help the human body to identify cells that are from its own body and also attack foreign cells that might have been introduced into the bloodstream. After identification, the ‘thing’ that defends our body from foreign invaders (antigens) is called an antibody.

Since antibodies defends only against foreign invaders, while leaving local cells intact, they must have the ability to recognize strangers. Our body takes care of this by producing antibodies for only one kind of antigen. For example, a Group A individual has only B-type antibodies to attack B-type blood and leave A-type blood intact. Similarly, a Group AB-type individual has neither A nor B type antibodies, since their bloodstream has both antigens. Otherwise, their body would start attacking its own blood. As you might guess, a Group O-type individual has both A and B-type antibodies.

How Blood is differentiated

How Blood is differentiated. Source- Wikipedia

Humans have another type of marker that characterizes their blood type in even greater detail. The Rh or the ‘Rhesus’ factor is a group of more than 40 antigens that also cover the surface of an RBC. An individual either has all of these antigens or none of them. Group A- signifies the presence of antigen A and an absence of Rh. Group B+ signifies the presence of antigen B as well as Rh. The name Rhesus comes from Rhesus monkeys, in which the protein was first discovered.MEME Blood Type Asian Dad

What is the significance of antigens during transfusions?

You might have heard the terms ‘universal donor’ and ‘universal recipient’ before. What these refer to is an individual’s ability to accept blood from another human.  If the blood types are not compatible, red blood cells will clump together, forming clots that can block blood vessels and cause death.

Since Group O- has none of the three antigens, but does have all three antibodies, their bodies react violently to a transfusion from a blood Group that might have any kind of antigen. Hence, they can only receive blood from another Group O- individual. The absence of antigens also gives them the ability to donate blood to anyone, so these individuals are called ‘Universal Donors’. Similarly, since Group AB+ people have all three antigens and none of the antibodies, so they can accept blood from any individual without risk. This makes them ‘Universal Recipients’.

Red Blood Cell Compatibility Chart

Red Blood Cell Compatibility Chart

Why do people have different blood types?

The reason why scientists believe that people have different blood type is evolution. Survival of a race is directly related to its ability to protect itself from diseases. Since the type of blood that a person has is completely based on their genetics, people in certain regions of the world have more of a particular blood type than the rest.

Blood Type Hereditary Chart

Blood Type Hereditary Chart

Being an inherited property, your blood group is based on the blood types of your parents. Hence, the place you live could statistically determine the possibility of you having a certain type. For example, malaria appears to be the main selective force behind type O, according to Christine Cserti-Gazdewich, a hematologist at Toronto General Hospital. Type O is more prevalent in Africa and other parts of the world that have high frequencies of malaria, suggesting that blood types carries some sort of evolutionary advantage. On the other hand, people with Type O blood may be more prone to other diseases. For example, they are known to be more susceptible to Helicobacter pylori, a bacterium that causes ulcers.

It is believed that the first antigen in the course of human evolution was Type A, but its susceptibility to smallpox virus and cancer led to its mutation into Type O. On the other hand, Type O is fairly susceptible to the bubonic plague, which might account for the reason why the frequency of Gene B is high in places that suffered epidemics of both the plague and smallpox, such as China, India and parts of Russia.

Distribution of Type A allele around the World

Distribution of Type A allele around the World. Source- antrho.palomar.edu

Although a concrete reason for the existence of ABO genes that determine blood types is not known, even after a hundred years of discovery, scientists believe that there must be an evolutionary advantage. Interestingly, in 1952 Bombay, doctors reported people that had none of the recognizable blood types normally found in humans. This phenomenon could mean that there is no evolutionary advantage after all, but these are very rare.  Claims have been made that people from group A have the worst hangovers and group O have the best teeth, for example, but these associations are probably coincidental at best.


  1. Blood Type – Wikipedia
  2. Mystery of Human Blood Types – Smithsonian.com
  3. Why do We Have Blood Types – Mosaicscience.com
  4. Human Blood Types – educreations (YouTube)

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