Why do humans have different blood group?

Knowing one’s blood group is very important.

It is asked at our schools, offices, etc, as something meant to be kept crucially in one’s medical records.

Every human has blood and externally all of it looks the same.

Then why do we have these variations in our blood groups and what do they signify?

What defines blood types?

Blood type varies mainly due to three important elements of blood.

These elements are the types of proteins, glycoproteins, and glycolipids.

These are expressed on the surface of red blood cells and define what type of blood type a person has. Moreover, other factors also play a role.

This is commonly found in the case of genes, as they are responsible for them, as they are inherited.

The pioneer in blood types Karl Landsteiner was the person to describe the original blood types in 1900. He defined 3 main types- A, B, and o.

Doctors in the current day and age recognize 23 blood group systems which further branch into hundreds of different types.

We have discovered many of the blood types and the antibodies that react to them as a result of transfusion incompatibilities. 
 
This can be simply explained as the fact that most people recognized the distinction due to the differences that came into place when people tried blood transfusions between people with different blood groups.

The advent of sophisticated biochemistry and molecular biology has helped to characterize a number of these entities.

Even though it has been recognized that the majority of the molecules are not considered essential for red cell function, some of them possess specific functions on red cell although it appears that the majority of the molecules are not essential for red cell function, some have specific functions on the red cell membrane such as allowing substances to enter and exit the red cell or binding certain substances to the cell surface.
 
Science has yet not come to a conclusion regarding the functions of the A and B blood group factors.

We have defined the O blood group as the absence of A and B blood group factors.

The most probable conclusion for this is in the way they are expressed on many of our cells and tissues in addition to blood cells.

These circulate in the plasma as well.

There are many statistical differences in the frequency of certain kinds of malignancies.

These correlate with ABO blood groups. 

Some facts about the blood groups

 It has been discovered that people with blood type A have about 20 percent greater risk of developing diseases like cancer of the stomach.

This is in comparison to individuals with Group O blood, who have a greater chance of developing ulcers in the future.

The causes behind this seem quite unclear.

Susceptibilities are unlikely to confer a significant survival advantage on a population, however, because (unlike malaria) they often occur beyond a subject's reproductive years. 
 
On the other hand, there is some evidence that group O members are more susceptible than other blood type individuals to the agent that causes bubonic plague, whereas group A people are more susceptible to the smallpox virus.

These correlations may account for the increased frequency of the B gene in China, India, and parts of Russia, which suffered epidemics of both of these diseases.

Infectious organisms that carry A- and B-like antigens may have indeed played a role in the somewhat different distribution of blood types worldwide.