The most popularly known blood grouping is the $\text{ABO}$ grouping. It is named $\text{ABO}$ and not $\text{ABC}$, because "O" refers to having:

1. A
   overdominance of this type on the genes for $\text{A}$ and $\text{B}$ types
2. B
   one antibody only — either anti-$\text{A}$ or anti-$\text{B}$ on the $\text{RBCs}$
3. C
   no antigens $\text{A}$ and $\text{B}$ on $\text{RBCs}$
4. D
   other antigens besides $\text{A}$ and $\text{B}$ on $\text{RBCs}$