Blood contains different cells with antigens that can cause an immune response if recognized as foreign. Blood transfusions must be checked for antigenic compatibility, with the ABO and rhesus systems commonly used. ABO typing depends on genes inherited from parents, while rhesus factor determines if blood is Rh positive or negative. Mismatched blood can cause serious illness or death.
Antigens are molecules that an organism’s body recognizes as foreign and targets for attack by the immune system. Blood contains different cells that can carry different antigens depending on a person’s genetic makeup. The introduction of blood recognized as foreign can cause serious illness, so the blood is always checked for antigenic compatibility before a transfusion. There are many different blood antigen typing systems, although only two, the ABO and the rhesus system, are commonly used in medicine.
Due to the fact that individuals have genes that are not exactly the same, one person’s physical makeup is very different from another. These differences also exist on a microscopic level. Blood, for example, contains different types of cells. Each of these cells is covered in various molecules that all perform specific functions.
A healthy person’s immune system recognizes the cells in their own body and knows that the molecules on the surface of those cells are harmless. When the immune system sees molecules it doesn’t recognize, however, it targets those molecules and any cells that display them for destruction. This is useful in situations such as infections, where this response can kill invading microbial cells. In the case of a blood transfusion, this can backfire.
It is the red blood cells in transfused blood that the immune system primarily controls. Although other types of blood cells, such as white blood cells, also display antigenic molecules on their surfaces, they do not attract a strong immune response. Therefore, the types of blood antigens mainly focus on those antigens displayed by red blood cells.
The most important of the blood antigenic systems is the ABO typing. This system focuses on a group of antigens that evoke a particularly strong immune response. These antigens are called A and B, and whichever type a person has depends on the genes of his or her parents.
If both a father and mother have a gene that tells the body to make a blood antigen, then their baby will have type A red blood cells. Two B genes and the baby has B blood. One A and one B gene mean the child has blood type AB. A person who inherits two genes that tell the body not to produce any of the antigens has type O blood. An A or B gene together with an O gene results in an A or a B, but never an O blood type.
Rhesus factor is another method of grouping red blood cells. In this case, a person may display rhesus factor antigen on the surface of red blood cells or not produce it at all. If it has it, the blood is Rh positive, and if it is absent, the blood is Rh negative.
Health problems can arise if blood transfused from one person to another is recognizable to the recipient as foreign. This will happen if the ABO type of donor blood contains antigens that are not already present in the recipient’s blood. Immune responses to mismatched blood can cause serious illness and even death.
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