Blood types are determined by the presence/absence of inherited antigens on the surface of red blood cells. These antigens may be proteins, glycoproteins, carbohydrates, or glycolipids.
There are four main blood groups:
- Type A blood has A antigens only on the surface (A+)
- Type B blood has both A and B antigens on RBCs (B-)
- O blood has no antigens (O-)
- AB blood has both A and B antigens (AB-).
Blood types are governed by genetics, and they result from a combination of genetic material from both parents.
Blood consists of red blood cells (which transport oxygen), white blood cells (which fight infection), and platelets (which help with blood clotting to stop/prevent bleeding). The type of antigens in our blood are also important in blood typing. There are at least 33 blood typing systems, but only two are widely used: ABO and Rh-positive/Rh-negative.
The A and B antigens are two important antigens present on the surface of red blood cells. These antigens are determined by genetics, allowing a person to be type A, type B, or type O.
Blood is also typed for the Rh factor, an antigen found in red blood cells. If the cells have the antigen, they are considered positive; if they do not have it, they are considered negative.
Blood types are assigned a positive or negative symbol based on whether the Rh antigen is present. A person’s blood type falls into one of these eight basic categories: 
World Population by Percentage of Blood Types
|Rank||Blood Type||Percentage of the World’s Population|
From the data above collected by World Atlas, O+ is the most common blood type in the world, while AB- is the rarest. 
Rarest Blood Type in the US
Source:  (please left click on graph and click EDIT DATA to get the values for this graph)
There are also considerable variations in blood types within ethnic groups in the USA. For instance, the Red Cross found that Asian Americans are much more likely than Latin Americans and white Americans to have a B-positive blood type. 
The American Red Cross defines rare blood types as those that occur in less than 1 in 1,000 people. The rarest of these is Rhnull.
People with rare blood types have a higher risk of complications during pregnancy, transfusion, and organ transplantation.
To make it clear for you, the following is the data presenting how many people in the USA have various blood types :
|Blood Type||How Many Have It|
|O +||1 person in 3|
|O –||1 person in 15|
|A +||1 person in 3|
|A –||1 person in 16|
|B +||1 person in 12|
|B –||1 person in 67|
|AB +||1 person in 29|
|AB –||1 person in 167|
O+ is one of the most common blood types, with about 37.4% of Americans having it. Individuals with O+ blood types can give blood to all Rh+ individuals (A+, B+, O+, and AB+) and receive blood from O-type individuals. 
O-type blood is one of the rarest types in the United States, comprising just 6.6% of the population of the USA. Moreover, it is universal, meaning it can be transfused to anyone. 
AB+ is one of the rarest blood group types in the USA, with only 3.4% of people having it. Moreover, AB+ is a universal recipient, meaning the person AB+ can receive blood products of any blood type. 
This one is the rarest blood type, with only 0.6% of the US people having it. AB- is a universal plasma donor, which means anyone can receive this blood type. 
B+ is another rarest blood type, with only 8.5% of the US population having it. People with this blood type can give blood products to AB+ and B+ and receive all O and B types. 
B- is one of the rarest blood types, with only 1.5% of the US population running it in its veins. People with this blood type can give blood products to all AB and B types as well as receive B- and O- types. 
A+ is one of the most common and most transfused blood types, with 35.7% of the US population having it. People with this blood type can give blood products to types AB+ and A+ as well as receive all O and A types. 
A- is one of the rarest blood types, with only 6.3% of the US population having it. People with this blood type can give blood products to all AB and A types as well as receive O- and A- types. 
Blood Type by Country/Population
People of Hispanic and African-American heritage are more likely to have an O blood type, while Asian-Americans are more likely to have a B blood type than other ethnicities within the US.
There are hundreds of antigens on every human’s red blood cell surface, some common and some rare. These antigens determine whether a person is compatible with another’s blood during transfusions, but most people do not need to worry about them unless they have a genetic blood disorder.
Following is the data of countries with the highest population having different types of blood types :
Countries with the Highest Percentage of People Having A+ Blood Type
|The Democratic Republic of the Congo||59.5%|
Countries with the Highest Percentage of People Having AB+ Blood Type
Regional Distribution of Blood Types
The O blood type is the most common in the United States, where it is carried by a majority of both Caucasians and African Americans. The O blood type is also the most common in Canada and South America.
The O blood type is prevalent in Africa; for example, Ghana, Libya, Congo, and Egypt have a high percentage of the population with O- blood types.
Though the O+ blood type is the most common in China, nearly 20% of the population has the B+ blood type. This blood type is also common in India and Central Asian countries.
Moreover, in some West Asian countries, such as Armenia and Azerbaijan, the A+ blood type is more common than in others.
The A blood group is predominant in European populations. For example, in Denmark, Norway, Austria, and Ukraine, the percentage of people with this blood type is as high as 40%.
Jamaica’s most common blood type group is B+, though nearly half of the country’s population has type O+.
The O+ and A+ blood types are common throughout the Pacific islands, with one exception: Fiji has a substantial population of the B+ blood type.
Approximately 41% of the Middle Eastern population has the blood type O+, with Lebanon being the only country with a strong O- and A- blood type population.
Blood typing systems use a complex set of criteria to categorize the many different blood types in humans. The ABO and Rh systems are the most commonly used blood typing systems, providing eight basic blood types.
Moreover, interpreting the data provided by World Atlas and Stanford Blood Center, we can conclude that AB- is the rarest blood type, while O+ is the most common one.
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