One caveat: while Rh is a protein, A and B are carbohydrate antigens.

For the most part, you can’t tell under the microscope a person’s blood type. However, some antigens, if they’re missing entirely, result in misshapen cells. For example, if missing all Rh antigens (not just the D that gives +/- as mentioned above) you get a shape called “stomatocyte”, where instead of a pale round area in the middle, it looks more like a closed mouth.