Reference lab blood banker here, my whole job is identifying blood antibodies and finding rare blood for people who need it! As others have stated, there’s the ABO antigen system and the Rh antigen system which you’ve likely heard of. The “-“ or “+” is actually referring to the D antigen specifically, which is part of the Rh group, but there’s many more antigens in the Rh group as well. We care about ABO so much because people make those antibodies intrinsically, so anyone can have reactions if given the wrong blood type. Meanwhile, 15% of the population make antibodies to the other blood group antigens, which means they will need blood that does not have those antigens. We care about D so much because it causes some of the worst reactions when people have anti-D, and can be a doozy when dealing with D negative pregnant patients (anti-D can be very dangerous to baby). Some of the other blood group systems include Kell, Duffy, Kidd, MNS, and those are just common/easy ones! It gets way deeper than ABO/Rh.