Blood type is a classification system based on the presence or absence of specific protein and carbohydrate markers, known as antigens, on the surface of red blood cells. These markers are genetically determined and act like identification tags for the immune system. Understanding blood type is important for two reasons: ensuring safety during medical procedures like blood transfusions and predicting how these traits are passed down within a family. This knowledge allows medical professionals to match blood correctly, preventing dangerous immune reactions, and helps manage genetic risks, especially during pregnancy.
The Basics of Blood Typing
The two main systems used to classify blood are the ABO system and the Rhesus (Rh) system, which combine to create the eight common blood types. The ABO system categorizes blood based on the presence of A and B antigens on the red blood cell surface. Type A blood has the A antigen, Type B has the B antigen, Type AB has both, and Type O has neither. A person’s plasma naturally contains antibodies against the antigens their red blood cells lack.
For example, a person with Type A blood has anti-B antibodies, while Type O blood contains both anti-A and anti-B antibodies. These antibodies develop within the first months and years of life. The Rh system is determined by the presence or absence of the D antigen, often called the Rh factor. If the D antigen is present, the person is Rh-positive (+); if it is absent, they are Rh-negative (-).
Unlike the ABO system, Rh-negative people do not naturally possess anti-D antibodies. They can develop these antibodies after exposure to Rh-positive blood, a process called sensitization. Combining these two systems creates the full blood type designation, such as O-positive or AB-negative.
Compatibility Rules for Transfusions
The primary danger in a blood transfusion is the recipient’s immune system reacting to the donor’s red blood cells. This reaction occurs when the recipient’s antibodies encounter foreign antigens, causing the red cells to clump together, a process known as agglutination. Agglutination can block blood vessels and lead to a life-threatening hemolytic transfusion reaction.
Type O-negative blood is the “universal donor” for red blood cells because it lacks A, B, and Rh (D) antigens. Since O-negative red cells have no surface markers for antibodies to attack, they can be safely given to any patient in an emergency. Conversely, Type AB-positive blood is the “universal recipient” because a person with this type has all three antigens, meaning their plasma contains no corresponding antibodies, allowing them to receive red blood cells from any type.
Despite these universal rules, the most common practice is to perform type-specific transfusions where donor and recipient types are precisely matched. Before any transfusion, cross-matching is performed by mixing a small sample of the donor’s red cells with the recipient’s plasma. This test confirms there is no agglutination, providing final verification of compatibility and reducing the risk of a reaction.
Genetic Rules of Blood Type Inheritance
Blood type is an example of Mendelian inheritance, controlled by one gene with three possible alleles: A, B, and O. Every person inherits one allele from each parent, resulting in two alleles that determine their blood type. The A and B alleles are co-dominant, meaning if both are inherited, they are fully expressed, resulting in Type AB blood. The O allele is recessive, so it is only expressed if a person inherits two copies of the O allele, resulting in Type O blood.
For instance, a person with Type A blood could have two A alleles (AA genotype) or one A and one O allele (AO genotype). This explains how two parents with Type A blood could still have a child with Type O blood, provided both parents have the AO genotype and pass on their recessive O allele. The Rh factor is inherited separately, determined by the presence or absence of the D antigen gene.
The Rh-positive trait is dominant, meaning a person will be Rh-positive if they inherit at least one Rh-positive allele. To be Rh-negative, a person must inherit the recessive Rh-negative allele from both parents.
Special Consideration for Rh Factor in Pregnancy
The Rh factor presents a unique concern when an Rh-negative mother is carrying an Rh-positive fetus, a scenario possible if the father is Rh-positive. During pregnancy, and especially during delivery, a small amount of the fetus’s Rh-positive red blood cells can cross the placenta and enter the mother’s bloodstream. The mother’s immune system recognizes the Rh factor as foreign and begins to produce anti-D antibodies, a process called Rh sensitization.
This initial exposure rarely affects the first Rh-positive fetus because sensitization often occurs late in pregnancy or during birth. However, once the mother is sensitized, these anti-D antibodies remain in her system and pose a risk to subsequent Rh-positive pregnancies. In a future pregnancy, these antibodies can cross the placenta and attack the fetus’s red blood cells, leading to hemolytic disease of the fetus and newborn.
This destruction of fetal red blood cells can cause severe anemia, jaundice, heart failure, and potentially stillbirth. To prevent this, Rh-negative mothers who have not yet been sensitized are given an injection of Rh immune globulin. This injection is administered around 28 weeks of gestation and again shortly after birth if the baby is confirmed to be Rh-positive, preventing the mother’s immune system from recognizing the fetal Rh antigens.