B1.1.7 Role of glycoproteins in cell–cell recognition Notes

Glycoproteins Are The “ID Badges” of Cells

1. Structure: The carbohydrate chains typically protrude from the cell surface, acting as identifiers for other cells.
2. Function
   1. Cell–Cell Recognition: Like ID badges, glycoproteins enable cells to recognize and distinguish each other as “self” or “non-self.”
   2. Tissue Organization: During embryonic development, glycoproteins ensure cells attach in the correct location to form tissues.
   3. Immune Defense: Immune cells use glycoprotein “barcodes” to detect and eliminate pathogens or infected cells.
   4. Blood Type Compatibility: Glycoproteins on RBCs determine blood groups (e.g., ABO system).

How Does Cell–Cell Recognition Work?

1. Carbohydrate “Barcodes”: The carbohydrate portion of glycoproteins is diverse, providing a unique marker for each cell type.
2. Receptor Interactions
   1. When one cell encounters another, receptor proteins on the first cell’s membrane bind to specific carbohydrate chains on the glycoprotein of the second.
   2. This lock-and-key interaction ensures high specificity in recognizing cell identity.

ABO Blood Group System: A Case Study in Glycoprotein Recognition

1. Antigens on RBCs
   1. Type O: Carries a base glycoprotein (antigen O).
   2. Type A: Has an N-acetylgalactosamine added to antigen O.
   3. Type B: Has a galactose added to antigen O.
   4. Type AB: Possesses both modifications (A and B).
2. Why It Matters
   1. Type O is called the “universal donor” since it lacks A or B modifications that might trigger an immune response.
   2. Type AB is the “universal recipient,” having both antigens and therefore no antibodies against A or B.
3. Immune Response
   1. People naturally produce antibodies against non-self antigens.
   2. Mismatched blood transfusions can lead to agglutination (RBC clumping) and severe complications.

How Blood Type Determines Compatibility

1. Type O Blood: Contains only antigen O and is often called the "universal donor" because it lacks the additional sugars (A or B) that could trigger an immune response.
2. Type A Blood :Contains antigen A. If transfused into someone without antigen A (e.g., type B or O individuals), the immune system will recognize it as foreign and reject the blood.
3. Type B Blood: Contains antigen B. Similarly, it will be rejected by individuals without antigen B (e.g., type A or O individuals).
4. Type AB Blood: Contains both antigens A and B and is often called the "universal recipient," as individuals with this blood type can accept blood from any group without triggering an immune response.

Why Does the Immune System Reject Foreign Blood?

1. The immune system produces antibodies that specifically target foreign antigens.
2. For example:
   1. A person with type A blood produces antibodies against antigen B.
   2. A person with type B blood produces antibodies against antigen A.
   3. A person with type O blood produces antibodies against both A and B antigens.
   4. A person with type AB blood does not produce antibodies against either A or B antigens.
3. When incompatible blood is transfused, these antibodies bind to the foreign antigens, causing red blood cells to clump together (a process called agglutination).
4. This can block blood vessels and lead to life-threatening complications.

Broader Implications of Glycoproteins in Recognition

1. Immune System Regulation
   1. Glycoproteins help immune cells detect aberrant cells (e.g., cancer, virus-infected).
   2. Errors in recognition can prompt autoimmune disorders.
2. Viral Infections: Certain viruses (e.g., HIV) attach to host glycoproteins to gain entry into cells.
3. Autoimmune Diseases: Misinterpretation of “self” glycoproteins can lead the immune system to attack normal tissues.

Conclusion: Glycoproteins as Gatekeepers of Cellular Communication

1. Glycoproteins serve a critical role in cell–cell interactions, enabling organisms to differentiate friends from foes, maintain tissue integrity, and control immune responses.
2. The ABO blood group system highlights how slight differences (e.g., one sugar attachment) can have major consequences (compatible vs. incompatible transfusions).