Expert: John Locke - 2/13/2010

Question
1. what is meant by the term RH positive?
2.how does RH incompatability affect blood transfusion

Answer
Thanks for using AllExperts. The Rh or Rhesus factor is a group of proteins that are found on the surface of human red blood cells and serve as a marker to identify those red blood cells to the immune system. One of the important controls on how the immune system works is the ability to recognize substances as being "self" or "non-self"; substances that are recognized as "self" are not subject to attack by the immune system, while "non-self" substances activate and are destroyed by the immune system. Various non-self entities include bacteria, viruses, protozoa, and inorganic materials that find their way into the body. Obviously the ability to differentiate in this way is critical to the immune system doing its job.

As I said above, the Rh factor system includes a number of these surface markers, of which the most important is the D factor. Referring to red blood cells (RBC) as Rh-positive or Rh-negative refers to the presence or absence of the D protein on their surface. Thus, there are two possibilities, and two possible immune reactions:

Rh-positive: The D factor is present on the surface of RBCs. If this blood is transfused into a person who is Rh-negative, their immune cells will see the D factor as foreign and become activated (the D factor is referred to as antigen, meaning it has the ability to cause an immune reaction). They will then attack and destroy the foreign RBCs, potentially causing the death of the person receiving the transfusion. People who are Rh-positive do not have antibodies against the D factor.

Rh-negative: The D factor is not present on the surface of RBCs. If this blood is transfused into a person who is Rh-positive, no immune response will occur. Speaking broadly, the immune system will recognize the presence of a foreign antigen and respond to that, but it will not recognize the absence of a self-antigen. Because of this, Rh-positive people can receive Rh-negative blood, but the reverse cannot occur. People who are Rh-negative produce antibodies against the D factor.

Rh incompatability, as I described above, occurs when Rh-positive blood is transfused to an Rh-negative person. Blood typing must control for this to avoid transfusion reactions; reactions of this kind can also occur between a pregnant woman and her fetus in a disease known as hemolytic disease of the newborn.