Everybody has a blood type. The most common blood type classification system is the ABO (say "A-B-O") system discovered by Karl Landsteiner in the early 1900s. There are four types of blood in the ABO system: A, B, AB, and O. Your blood type is established before you are born, by specific genes inherited from your parents. You receive one gene from your mother and one from your father; these two combine to establish your blood type. These two genes determine your blood type by causing proteins called agglutinogens (a-GLOO-tin-a-gins) to exist on the surface of all of your red blood cells.
There are three alleles or versions of the blood type gene: A, B, and O. Since everybody has two copies of these genes, there are six possible combinations; AA, BB, OO, AB, AO, and BO. In genetic terms, these combinations are called genotypes, and they describe the genes you got from your parents.
In addition to the proteins (agglutinogens) existing on your red blood cells, other genes make proteins called agglutinins (a-GLOO-tin-ins) that circulate in your blood plasma. Agglutinins are responsible for ensuring that only the blood cells of your blood type exist in your body.
Your genotype determines your blood type.
The agglutinogen produced by the O allele has no special enzymatic activities. However, the agglutinogens produced by the A and B alleles do have enzymatic activities, which are different from each other. Therefore people whose genotype is OO are said to have type O blood, meaning the agglutinogen on their red blood cells doesn't have any enzymatic activity. People with Type O blood have agglutinins a and b in their blood plasma. Agglutinin a helps the body destroy any type A blood cells that might enter the circulation system. Agglutinin b helps the body destroy any type B blood cells that might enter the circulation system.
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