Our red blood cells have different proteins on their surfaces, the main ones being antigen A, B and Rh (Rh antigen). Detection of these antigens is at the basis of each individual’s blood type (A, B, AB or O), and each type may be negative or positive depending on the absence or presence of the Rh antigen. In contrast, the body develops antibodies against antigens that are absent from its red blood cells: a person with type A has anti-B antibodies and vice versa. People with type AB do not have anti-A or anti-B antibodies. The body does not normally produce anti-Rh antibodies. These antibodies only develop upon contact with “foreign” red blood cells, either as a result of a transfusion or in an Rh-negative mother who is carrying an Rh-positive fetus.
These factors are extremely important when a person needs a blood transfusion or marrow transplant, or during pregnancy to determine the Rh status. The O-negative blood type is called a “universal donor” while the AB-positive type is called a “universal recipient.”
Given the much more sensitive techniques in use, it is not impossible for individuals once considered Rh-negative to see their status changed to Rh-positive.