Medically reviewed by Amelia MacIntyre, DO One of the most important characteristics of blood is its type. Knowing your blood ...

A negative. If you have this rare blood type (like 6.3% of the U.S. population), you can donate blood to anyone with A or AB blood type. B positive. Another rare type, it is found in 8.5% of people.

It's a bit simplistic, but you can think of the Rh factor as a protein that is either present (positive) or absent (negative) on red blood cells. Exact percentages vary with race, but most people ...