Likewise, people with the BB and the BO genotypes are said to have type B blood. These people have agglutinin a in their blood plasma. Agglutinin a helps the body destroy any type A blood cells that might enter the circulation system.
People who have the AB genotype have the enzyme activity associated with both the A and B alleles. These people have no agglutinins in their blood plasma.
The concepts of genotype and phenotype can be easily understood in the case of blood type. Genotype refers the actual genes an individual possesses that determine a particular trait. Phenotype refers to the characteristics of that trait an individual displays. In the case of blood type, both the AA and AO genotypes cause individuals to display the A blood type phenotype. Similarly, both the BB and BO genotypes cause individuals to display the B blood type phenotype. Individuals who are phenotypically type O or type AB have only one possible genotype, OO and AB, respectively.
In different parts of the world, the fraction of individuals with blood type A, B, O, or AB differs. The frequency with which blood types are observed is determined by the frequency with which the three alleles of the ABO gene are found in different parts of the world (allele frequency). Variation in the allele frequency at the ABO gene reflects the social tendency of populations to marry and reproduce within a national, regional, or ethnic group. As people throughout the world intermingle to a greater extent, the distribution of the different blood types will become more uniform throughout the world.
Blood Transfusions
Because there are only four types of blood, it is possible to take blood from one person and donate it to another person in a process called transfusion. In order for a transfusion to work it is essential that the agglutinogens on the surface of the donor's blood cells match the agglutinogens on the surface of the recipient's blood cells. In other words, the blood type of the donor and the blood type of the person receiving the transfusion must be compatible. If the blood types don't match, special antibodies in the recipient's blood, called agglutinins, will attack the donated blood causing blood clots to form in a reaction called agglutination.
If you ever need a blood transfusion, someone will take a sample of your blood in order to determine your blood type and the genotypes of matching blood types. Someone with type A blood can receive blood from people with the AA, AO, and OO genotypes. People with type B blood can receive blood from people with the BB, BO, and OO genotypes.
There are two special genotypes when it comes to blood transfusions: OO and AB.
The first special genotype is OO. People with O blood are said to be universal donors because they can donate blood to everybody. However, people with type O blood can only receive transfusions from other type O donors. Because O blood does not carry either the A or B agglutinogens, the immune system of a person with O blood views these agglutinogens as foreign. People with type O blood have agglutinins in their plasma that will react against the A and B agglutinogens. Therefore, type O blood will undergo agglutination if exposed to A, B, or AB blood.
