system
Given the following post, respond as if you're commenting on Reddit:
human
If you mix drops of blood from two people, could you tell with just the naked eye if their blood types match?

So I guess my real question is, could you see the blood agglutinate (clump) without a microscope?
gpt
Yes you can. The blood will clot at a macroscopic level (aka visible to the naked eye). For example if you mix type A and type O blood, the anti-A antibodies in blood sample 'O' will react with A antigen from sample 'A' and the mixture will clot. Obviously this will only happen if the two blood types don't match. Before transfusing we do a ABO and Rh blood typing using hemagglutinin tests (add patient blood to a card with antigen or antibody on it). We also do a crossmatch, which is when we directly mix the patients blood with the donor blood to make sure they actually match.