The most popularly known blood grouping is the $\mathrm{ABO}$ grouping. It is named $\mathrm{ABO}$ and not…
The most popularly known blood grouping is the $\mathrm{ABO}$ grouping. It is named $\mathrm{ABO}$ and not $A B C$, because ' $O$ ' in it refers to having
other antigens besides A and B on RBCs
over dominance of this type on the genes for A and $B$ types
one antibody only-either anti-A or anti-B on the RBCs
no antigens $A$ and $B$ on RBCs
Solution
Landsteiner divided human population into four groups based on the presence of antigens found in their RBCs. Each group represented a blood group. Thus, there are four types of blood groups A, B, AB and O. Blood group ' $O$ ' does not contain any antigen on RBCs, hence can be given to any person, that's why this blood group is called universal donor.