Red blood cells have surface-expressed proteins that define the self/not-self nature of the cells.
- Evaluate the ABO and Rhesus blood groups in terms of donors and recipients
- Surface-expressed proteins called antigens on red blood cells determine an individual’s blood type. There are two types of antigen groups: the ABO system antigens and a Rhesus D antigen.
- Exposure to a blood group antigen that is not recognized as self will cause the immune system to make specific antibodies to the new blood group antigen, often leading to destruction of the cells.
- Knowing an individual’s antigen type is important to ensure compatibility if a transfusion is needed.
- Blood type is inherited. O type is the most common despite being a recessive gene because it is more highly expressed in the gene pool, while type A and type B are dominant (and type AB is codominant) but are less common because they are less expressed in the gene pool.
- Individuals may also be positive or negative for the rhesus D antigen in addition to their blood type. Rhesus D complications are common during fetal development if the parents differ in rhesus antigen expression.
- antibodies: Also known as an immunoglobulin (Ig), a large Y-shaped protein produced by B-cells that is used by the immune system to identify and neutralize foreign objects such as bacteria and viruses.
- antigen: A substance that induces an immune response, usually foreign.
Red blood cells have surface-expressed proteins that act as antigens, which are molecules that can illicit an immune system response. Red blood cells belong to different groups on the basis of the type of antigen that they express. Blood type determines compatibility for receiving blood transfusions from other people.
The ABO Blood Group System
If an individual is exposed to a blood group antigen (A or B) that is not recognized as self, the individual can become sensitized to that antigen. This will cause the immune system to make specific antibodies to a particular blood group antigen and form an immunological memory against that antigen. These antibodies can bind to antigens on the surface of transfused red blood cells (or other foreign tissue cells), often leading to destruction of the cells by recruitment of other components of the immune system.
Knowledge of an individual’s blood type is important to identify appropriate blood for transfusion or tissue for organ transplantation. There are four blood types that differ based on the antigen expressed by the red blood cell and by the type of associated antibody found in the plasma. The type of antigen determines which blood types that blood type may safely be donated to, while the type of antibody determines which types of antigen (and types of blood) will be rejected by the body.
Blood type classification: Blood type (or blood group) is determined, in part, by the ABO blood group antigens present on red blood cell.
- Blood group A individuals have the A antigen on the surface of their RBCs, and blood serum containing IgM antibodies against the B antigen. Therefore, a group A individual can only receive blood from individuals of groups A or O types, and can donate blood to individuals of groups A or AB.
- Blood group B individuals have the B antigen on their surface of their RBCs, and blood serum containing IgM antibodies against the A antigen. Therefore, a group B individual can only receive blood from individuals of groups B or O, and can donate blood to individuals of groups B or AB.
- Blood Group AB individuals have both A and B antigens on the surface of their RBCs, and their blood serum does not contain any antibodies against either A or B antigen. Therefore, an individual with type AB blood can receive blood from any group, but can only donate blood to another group AB individual. AB blood is also known as “universal receiver.”
- Blood group O individuals do not have either A or B antigens on the surface of their RBCs, but their blood serum contains IgM antibodies against both A and B antigens. Therefore, a group O individual can only receive blood from a group O individual, but they can donate blood to individuals of any ABO blood group (i.e. A, B, O, or AB). O blood is also known as “universal donor.”
Blood types are inherited and represent genetic contributions from both parents. The gene that codes for blood type contains three alelles: IA and IB which give type A and B blood and are dominant, and i, which is recessive and codes type O. Children will have blood types similar to their parents based on inheritance. The i allele is far more commonly expressed in the gene pool than IA and IB, which is why type O blood is the most common type despite being a recessive phenotype. Type AB is the rarest because it is the combination of less commonly expressed alleles, and is the result of codominance between IA and IB alelles.
Many people also have the rhesus D (Rh) antigen expressed by their red blood cells. Those that are have Rh antigens are positive for Rh, while those that don’t have it are Rh negative (ie. type O+ is type O with rhesus, type A- is type A without rhesus). Rh positive individuals do not have the antibodies for the Rh factor, but can make them if exposed to Rh. Besides being a consideration for blood transfusion, parents who differ based on Rh status must be cautious to ensure that maternal antibodies do not destroy their child’s red blood cells during fetal development, which can cause hemolytic anemia.