The A negative blood type is only present in 6.3 percent of the human population. The A portion of the blood type means that the red blood cells have attached A antigens, and the negative portion means that the red blood cells do not have the Rh factor antigen.
Blood type is determined by the presence or absence of certain antigens on red blood cells. The two most common systems of blood typing are ABO and Rh typing. ABO blood typing looks at the presence or absence of A and B antigens on the surface of red blood cells. Type A blood contains the A antigen without the B antigen, and subsequently a type A person's body produces anti-B antibodies that recognize any B antigens as non-self, or foreign, and attack them. Type O blood contains neither A nor B antigens, so it has anti-A and anti-B antibodies.
Rh blood typing looks at presence or absence of a specific Rh factor in the blood. Negative means that the blood does not contain the Rh factor, so the body of a person with Rh-negative blood produces anti-Rh antibodies to attack any Rh factor that is found in the blood, which would occur if an Rh-negative individual received a blood transfusion of Rh-positive blood.