Antigens

An antigen is any antigen substance that, when introduced into a body and recognized as foreign, will bring about an immune response. This might result in the production of an antibody that will react specifically with that antigen in some observable way.

Blood group antigens

Blood is classified into different groups according to the presence or absence of molecules called antigens on the surface of every red blood cell in a person's body. 

Antigens determine blood type and can either be proteins or complexes of sugar molecules. The genes in the blood group antigen group provide instructions for making antigen proteins. 

Blood group antigen proteins serve a variety of functions within the cell membrane of red blood cells. These protein functions include transporting other proteins and molecules into and out of the cell, maintaining cell structure, attaching to other cells and molecules, and participating in chemical reactions.

Blood group antigens play a role in recognizing foreign cells in the bloodstream. For example, if a person with blood type A receives a blood transfusion with blood type B, the recipient's immune system will recognize the type B cells as foreign and mount an immune response. 

Antibodies against type B blood cells (anti-B antibodies) are made, which attack and destroy the type B blood cells. This sort of blood type mismatch can lead to illness. Some blood types are associated with more severe immune reactions than others. 

The blood type of donated cells, or tissues in the case of organ donation, is checked before being given to a recipient to prevent this immune response.