What is a blood antigen? (with photos)

Two packs of O blood, which can be transfused to anyone.

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 that is recognized as foreign can result in serious illness and therefore blood is always checked for antigenic compatibility before a transfusion. There are many different blood antigen typing systems, although only two, the ABO system and the rhesus system, are commonly used in medicine.

The set of blood types Rhesus factors.

Because individuals have genes that are not exactly alike, one person’s physical makeup is very different from another’s. These differences exist even on a microscopic level. Blood, for example, contains many different types of cells. Each of these cells is covered by several molecules that perform specific functions.

A healthy person’s immune system 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 it doesn’t recognize, however, it targets those molecules and any cells that display them for destruction. This is useful in situations like infections, where this response can kill invading microbial cells. In the case of a blood transfusion, it can backfire.

The antigens displayed by red blood cells are the main focus of blood antigen typing.

It is the red blood cells from the transfused blood that the immune system primarily checks. 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, blood antigen types mainly focus on the antigens displayed by red blood cells.

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Blood is always checked for antigenic compatibility before a transfusion.

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 any type of person depends on the genes of their parents.

If the father and mother both have a gene that tells the body to produce blood antigen A, then their child will have type A red blood cells. Two B genes and the child has B blood. An A and a B gene means that the child have type AB blood. A person who inherits two genes that tell the body not to produce any of the antigens has Blood Type O. An A or B gene together with an O gene results in an A or B, but never an O blood type.

Rhesus factor is another method of grouping red blood cells. In this case, a person may display the Rhesus factor antigen on the surface of red blood cells or simply not produce it. If it has, the blood is Rh positive, and if absent, the blood is Rh negative.

Medical problems can arise if blood transfused from one person to another is recognized 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 mismatched blood can cause serious illness and even death.

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