The red blood cells of humans have one of four different antigens (proteins) on their surface: A, B, AB, or O. The “type” of blood you have is determined by which antigen is present on your cells. If you have “A” antigens, you are type A blood, if you have “O” antigens, you’re type “O,” and so on.
Blood types are further categorized by the presence (or absence) of another antigen on the red blood cells, the Rhesus (Rh) factor. Those with Rhesus present on their cells are said to be Rh positive, while those lacking the protein are said to be Rh negative. About 15% of Americans lack the Rh factor, so most people in the United States have “positive” blood.
Type O negative blood is the universal donor because they can donate to any other blood type.
Type AB+ is the universal recipient because they can receive blood from any other blood type.