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Blood type is determined by the presence or absence of certain antigens on the surface of red blood cells. These antigens are proteins or carbohydrates that are inherited from the parents. There are many different blood types, but the most common ones are A, B, AB, and O.

People with type A blood have the A antigen on their red blood cells. People with type B blood have the B antigen. People with type AB blood have both the A and B antigens. People with type O blood do not have any of the A or B antigens.

If a person receives a blood transfusion with blood that has a different blood type than their own, their immune system will attack the foreign blood cells. This can lead to a serious reaction called a transfusion reaction.

That is why it is important to know your blood type and to only receive blood transfusions from people with the same blood type.

Here are some of the reasons why people have different blood types:

• Genetics: Blood type is inherited from the parents. Each person has two genes for blood type, one from each parent. The possible combinations of these genes are A, B, AB, and O.
• Mutations: Sometimes, there can be mutations in the genes that control blood type. This can lead to rare blood types that are not inherited from the parents.
• Infections: Some infections can also affect blood type. For example, people with malaria often have a different blood type than they were born with.

The exact reason why people have different blood types is not fully understood, but it is likely due to a combination of genetics, mutations, and infections.