system
Given the following post, respond as if you're commenting on Reddit:
human
EL5 the differences in blood types?

I'm a blood donor, and found out that I'm O+. Idk what that means and Wikipedia isn't helping.
gpt
Basically there are 8 blood types, defined by the presence of specific antigens on the surface of red blood cells. There are 4 main types separated by letter: A(has the A antigen), B(has the B antigen), AB(has both A and B), and O(has neither A nor B). Additionally, there is a 3rd antigen called the Rh factor which determines the + or -. 

As far as Blood donations go, put simply, a given type of blood can be given only to people who have at least that level of antigens. For example, type B+ can only be given to people with either B+ or AB+ because those are the only blood types with both the B antigen and Rh factor. 

This means that O- (which has none of the 3 antigens) can be given to all blood types, which is why it's called the universal donor. Similarly, AB+ (which has all 3 antigens) can receive all blood types, which is why it's called the universal recipient.

Of course there are additional factors such as ethnicity that can play a role in the choice of blood to reduce the likelihood of a bad reaction, but this is the basic idea.