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Our blood is composed of blood cells and an aqueous fluid known as plasma. Human blood type is determined by the presence or absence of certain identifiers on the surface of red blood cells. These identifiers, also called antigens, help the body's immune system to recognize its own red blood cell type.
There are four main ABO blood type groupings: A, B, AB, and O. These blood groups are determined by the antigen on the blood cell surface and the antibodies present in the blood plasma. Antibodies (also called immunoglobulins) are specialized proteins that identify and defend against foreign intruders to the body. Antibodies recognize and bind to specific antigens so that the foreign substance can be destroyed.
Antibodies in an individual's blood plasma will be different from the antigen type present on the red blood cell surface. For example, a person with type A blood will have A antigens on the blood cell membrane and type B antibodies (anti-B) in the blood plasma.
ABO Blood TypesABO blood group antigens present on red blood cells and IgM antibodies present in the serum. InvictaHOG/Wikimedia Commons/Public Domain Image
While the genes for most human traits exist in two alternative forms or alleles, the genes that determine human ABO blood types exist as three alleles (A, B, O). These multiple alleles are passed from parent to offspring such that one allele is inherited from each parent. There are six possible genotypes (genetic makeup of inherited alleles) and four phenotypes (expressed physical trait) for human ABO blood types. The A and B alleles are dominant to the O allele. When both inherited alleles are O, the genotype is homozygous recessive and the blood type is O. When one of the inherited alleles is A and the other is B, the genotype is heterozygous and the blood type is AB. AB blood type is an example of co-dominance since both traits are expressed equally.
- Type A: The genotype is either AA or AO. The antigens on the blood cell are A and the antibodies in the blood plasma are B.
- Type B: The genotype is either BB or BO. The antigens on the blood cell are B and the antibodies in the blood plasma are A.
- Type AB: The genotype is AB. The antigens on the blood cell are A and B. There are no A or B antibodies in the blood plasma.
- Type O: The genotype is OO. There are no A or B antigens on the blood cell. The antibodies in the blood plasma are A and B.
Due to the fact that a person with one blood type produces antibodies against another blood type when exposed to it, it is important that individuals be given compatible blood types for transfusions. For example, a person with blood type B makes antibodies against blood type A. If this person is given blood of type A, his or her type A antibodies will bind to the antigens on the type A blood cells and initiate a cascade of events that will cause the blood to clump together. This can be deadly as the clumped cells can block blood vessels and prevent proper blood flow in the cardiovascular system. Since people with type AB blood have no A or B antibodies in their blood plasma, they can receive blood from persons with A, B, AB, or O type blood.
Rh FactorBlood Group Test. MAURO FERMARIELLO/Science Photo Library/Getty Images
In addition to the ABO group antigens, there is another blood group antigen located on red blood cell surfaces. Known as the Rhesus factor or Rh factor, this antigen may be present or absent from red blood cells. Studies performed with the rhesus monkey lead to the discovery of this factor, hence the name Rh factor.
Rh Positive or Rh Negative: If the Rh factor is present on the blood cell surface, the blood type is said to be Rh positive (Rh+). If absent, the blood type is Rh negative (Rh-). A person who is Rh- will produce antibodies against Rh+ blood cells if exposed to them. A person can become exposed to Rh+ blood in instances such as a blood transfusion or a pregnancy where the Rh- mother has an Rh+ child. In the case of an Rh- mother and Rh+ fetus, exposure to the blood of the fetus can cause the mother to build up antibodies against the child's blood. This can result in hemolytic disease in which fetal red blood cells are destroyed by antibodies from the mother. To prevent this from happening, Rh- mothers are given Rhogam injections to stop the development of antibodies against the blood of the fetus. Like the ABO antigens, the Rh factor is also an inherited trait with possible genotypes of Rh+ (Rh+/Rh+ or Rh+/Rh-) and Rh- (Rh-/Rh-). A person who is Rh+ can receive blood from someone who is Rh+ or Rh- without any negative consequences. However, a person who is Rh- should only receive blood from someone who is Rh-.
Blood Type Combinations: Combining the ABO and Rh factor blood groups, there are a total of eight possible blood types. These types are A+, A-, B+, B-, AB+, AB-, O+, and O-. Individuals who are AB+ are called universal recipients because they can receive any blood type. Persons who are O- are called universal donors because they can donate blood to persons with any blood type.