What are the four major human blood types?

There are four common blood types in the human population, based on the presence or absence of specific antigens on the surface of red blood cells:**

- Blood Type A: This blood type has only the A antigen on the red blood cells. People with blood type A can only receive blood from individuals with blood type A or O, and they can donate blood to individuals with blood type A or AB.

- Blood Type B: Blood type B has only the B antigen on the red blood cells. People with blood type B can only receive blood from individuals with blood type B or O, and they can donate blood to individuals with blood type B or AB.

- Blood Type AB (universal recipient): This blood type has both A and B antigens on the red blood cells. People with blood type AB can receive blood from any of the four blood groups without any issues, but they can only donate blood to other individuals with blood type AB.

- Blood Type O (universal donor): Blood type O does not have any A or B antigens on the red blood cells. This is considered the "universal" donor type, as people with blood type O can donate blood to individuals of any blood group. However, they can only receive blood from other individuals with blood type O.

It's worth mentioning that in addition to these four major blood types, there are also variations based on the presence of other antigens and antibodies, which is why blood transfusions require careful matching to ensure compatibility.