Often abbreviated ABO, blood types are inherited and fall into four groups or types: O, A, B, AB. Each blood type also is identified as either Rh positive or negative (the Rh factor being an inherited blood group on red blood cells). Approximately 85% of the U.S. population is Rh-positive (i.e., O+, A+, B+, AB+). Those who do not have the Rh factor are Rh-negative (O-, A-, B-, AB-) and are quite rare.
| O+ | 38% | |
| O- | 7% | Universal red cell donors |
| A+ | 34% | |
| A- | 6% | |
| B+ | 9% | |
| B- | 2% | |
| AB+ | 3% | Universal plasma donor and red cell recipient |
| AB- | 1% | Universal plasma donor |