Why do we have different blood types?

Why do we have different blood types?

How many blood types in the world? We may know that blood types are divided into A, B, AB, and O. So, why do we have different blood types? Although our blood’s color are same. Before the 20th century, people thaught that that there’s no different between my blood and your blood. And because of that, a lot of people died because they got wrong blood transfusion. After that, a researcher named Karl Landsteiner found that human’s blood are divided into A, B, and O. In general, we can have different blood types because our body have something called “antigen”. Someone who has antigen A, will have blood type A and so on. But, people who have antigen A and B will have blood type AB. Our immune system then will read our antigen in our blood cells. And if our immune system find something different from the else, they will consider it as something dangerous that could harm our body. And so, they will fight it. The reaction of resistance will cause blood clots and can killed people.