"Furthermore, ABO blood types are inherited as alleles (one from each parent), and they display patterns of dominant and codominant inheritance. The alleles for A and B blood types are codominant to each other, and both are dominant over blood type O. Therefore, individuals with genotypes of AA or AO have type A blood and express the A carbohydrate antigen on the surface of their RBCs. People with genotypes of BB or BO have type B blood and express the B carbohydrate antigen on the surface of their RBCs. Those with a genotype of AB have type AB blood and express both A and B carbohydrate antigens on the surface of their RBCs. Finally, individuals with a genotype of OO have type O blood and lack A and B carbohydrates on the surface of their RBCs. It is important to note that the RBCs of all four ABO blood types share a common protein receptor molecule, and it is the addition of specific carbohydrates to the protein receptors that determines A, B, and AB blood types. The genes that are inherited for the A, B, and AB blood types encode enzymes that add the carbohydrate component to the protein receptor. Individuals with O blood type still have the protein receptor but lack the enzymes that would add carbohydrates that would make their red blood cell type A, B, or AB. IgM antibodies in plasma that cross-react with blood group antigens not present on an individual’s own RBCs are called isohemagglutinins (figure 1.45). Isohemagglutinins are produced within the first few weeks after birth and persist throughout life. These antibodies are produced in response to exposure to environmental antigens from food and microorganisms. A person with type A blood has A antigens on the surface of their RBCs and will produce anti-B antibodies to environmental antigens that resemble the carbohydrate component of B antigens. Those with type B blood have B antigens on the surface of their RBCs and will produce anti-A antibodies to environmental antigens that are similar to the carbohydrate component of A antigens. People with blood type O lack both A and B antigens on their RBCs and, therefore, produce both anti-A and anti-B antibodies. Conversely, people with AB blood type have both A and B antigens on their RBCs and, therefore, lack anti-A and anti-B antibodies. Figure 1.45: Blood types. Figure description available at the end of the chapter."