Antigens are protein or carbohydrate molecules found on the surface of red blood cells, white blood cells, platelets, and other body tissues. They are markers that tell the immune system that something is foreign and can trigger the immune system to produce antibodies directed against the donor’s blood group antigens if the recipient does not have that antigen. Antibodies are proteins found in plasma that recognize foreign substances and alert the immune system to destroy them.

In the context of blood transfusions, the antigens expressed on the red blood cell determine an individuals blood group. The ABO system is the most well-known blood group system, which has four main blood groups: A, B, AB, and O. Blood group antigens are either sugars or proteins, and they are attached to various components in the red blood cell membrane. For example, the antigens of the ABO blood group are sugars, and a persons DNA determines the type of enzymes they have, and therefore, the type of sugar antigens that end up on their red blood cells.

If a patient receives blood transfusions, their immune systems will attack any donor red blood cells that contain antigens that differ from their self-antigens. Therefore, ensuring that the antigens of transfused red blood cells match those of the patients red blood cells is essential for a safe blood transfusion. Blood group antigens can be sugars or proteins, and they are attached to various components in the red blood cell membrane.