Sure, but I’m not an expert by any stretch of the imagination. So the cell is surrounded by a membrane. That membrane is absolutely covered in tons and tons and tons of proteins which stick out above the surface of the membrane. We call these surface proteins because they extend above the surface. Those proteins can be used to mark what type of cell the cell is, interact with chemicals floating around outside the cell, bind the cell to various structural components or other cells etc. These are sometimes called surface antigens, because antibodies can be bind to them.

The normal blood group markers, the ABO system, represent a set of surface antigens. The ABO and Rh (that’s the +/- part) happen to be particularly important to how the body recognizes self vs foreign cells. So it’s really important to match those correctly.

But the blood cells have hundreds of different antigens. And just because the ABO and Rh groups are particularly important for recognition doesn’t mean those other antigens aren’t also used. And to make it even more complicated, different people seem to respond more or less strongly to different groups (although ABO seems to be close to universally important)

Here’s an article I found on the topic