Glycoproteins act as cell-surface identification markers: their carbohydrate chains allow cells and the immune system to recognize particular cell types. In the ABO system, surface antigens identify red blood cells as group A, B, AB or O.
This content is required at both SL and HL in B1.1 Carbohydrates and lipids.
A glycoprotein consists of a protein with an attached carbohydrate chain. In the plasma membrane, the protein is embedded in the membrane while the carbohydrate projects from the extracellular surface, where it can interact with receptors or antibodies.
The precise structure of the carbohydrate chain acts like a molecular label. In ABO blood grouping, red blood cells display A antigens, B antigens, both, or neither. These markers may occur on membrane glycoproteins and glycolipids.
| Blood group | Antigens on red blood cells | Antibodies in plasma |
|---|---|---|
| A | A antigen | Anti-B |
| B | B antigen | Anti-A |
| AB | A and B antigens | Neither anti-A nor anti-B |
| O | Neither A nor B antigen | Anti-A and anti-B |
If an antibody encounters its complementary antigen, it binds to it and may cause agglutination, in which red blood cells clump together. For example, giving group A blood to a person with group B blood allows the recipient’s anti-A antibodies to bind to the donor’s A antigens. This can produce a dangerous transfusion reaction.
A common misconception is that antigens and antibodies are the same. An antigen is a recognition marker on the red blood cell surface, whereas an antibody is a plasma protein that binds specifically to an antigen.
Exam technique: For an “explain” question, link structure to function: carbohydrate chains project from the membrane → their shapes differ → antibodies or receptors recognize specific shapes → cells can be identified. When applying this to transfusions, name both the donor-cell antigen and the recipient’s corresponding antibody.
