Under the common ABO blood typing process, a person's blood type could be A, B, AB or O. It is very important for people to know their own blood type, as well as that of their spouse and children. Important decisions concerning emergency blood transfusions may have to made quickly, so having this information on hand for medical professionals can shave precious minutes off the initial triage process following a trauma. A person's blood type is determined largely by genetics, and it does not change through his or her lifetime. A simple blood typing test can be performed literally anywhere by anyone through the use of specially-treated testing cards.
One of the main factors that determines blood type is family genetics. A child receives separate sources of genetic code, called alleles, from each parent at the time of conception. One of the alleles located on chromosome 9 contains the precise type of the donor parent, and is classified as A, B, AB or O. An additional factor is called the Rhesus factor, which could be positive or negative. The actual blood type of a child is determined by the dominant type between the two parents. A and B are both dominant over O, which means a child that receives an A from the father and an O from the mother will have a type A blood.
Subsequently, A and B are considered to be codominant, which means a child inheriting an A from the mother and a B from the father will most likely have an AB blood type. Only two recessive O genes from both parents will result in a child having type O. An O negative blood type is considered to be a universal donor, since it contains nothing that would appear foreign to someone else's blood. Those with type A or B positive must not receive blood infusions of the opposite type, since the body's natural defenses will attack the incoming blood cells as they would any other infection.
A person's blood type is determined through a simple ABO test available at a doctor's office, blood donation center, or even through pharmacies. A drop of blood is placed on two separate testing circles marked A and B. The card has already been prepared with dried serum containing anti-A and anti-B chemicals. If the blood reacts to the A circle but not the B circle, then the tester's blood type is considered to be A. A reaction to both circles indicates type AB, while a complete non-reaction to either circle indicates type O blood. The reaction is caused by the chemicals on the card coming in contact with type A or type B antigens on the surface of the red blood cells.