Antigens are molecules that the body of an organism recognizes as foreign and targets for attack by the immune system. Blood contains different cells that can carry different antigens depending on the genetic makeup of the person. Introducing blood that is recognized as foreign can result in serious illness, and so blood is always checked for antigenic compatibility before a transfusion. Many different blood antigen typing systems exist, 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, the physical makeup of one person is very different to another. These differences even exist at a microscopic level. Blood, for example, contains several different types of cells. Each of these cells is covered in various molecules that all perform specific functions.
The immune system in a healthy person recognizes the body's own cells and knows that the molecules on the surface of those cells are harmless. When the immune system sees molecules that 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 the transfused blood that the immune system primarily checks out. Although other types of blood cells, such as the white blood cells, also display antigenic molecules on their surfaces, these do not attract an intense immune response. Therefore, blood antigen types primarily focus on those antigens displayed by the red blood cells.
The most important of the blood antigen systems is 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 whatever type a person has depends on the genes from his or her parents.
If both a father and a mother have a gene that tells the body to produce A blood antigen, then their child will have Type A red blood cells. Two B genes, and the kid has B blood. One A and one B gene means the child has AB-type blood. A person who inherits two genes that tell the body to produce neither of the antigens has Type O blood. One A or B gene along with one O gene results in either 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 can either display Rhesus factor antigen on the surface of red blood cells or not produce it at all. If he or she has it, the blood is Rhesus positive, and if absent, the blood is Rhesus negative.
Medical problems can arise if the blood transfused from one person to another is recognizable as foreign by the recipient. This will happen if the ABO type of the donor's blood contains antigens that are not already present in the recipient's blood. Immune responses to the mismatching blood can cause severe illness and even death.