What is the Difference Between Antigen A and Antigen B

The main difference between antigen A and antigen B is that the immunodominant sugar of antigen A is N-acetyl-D-galactosamine whereas the immunodominant sugar of antigen B is D-galactose. Furthermore, antigen A occurs in A and AB blood groups, while antigen B occurs in B and AB blood groups. 

Antigen A and antigen B are the two types of blood group antigens which determine the blood group of the ABO blood group system. Generally, they are oligosaccharide antigens. 

Key Areas Covered 

1. What is Antigen A
     – Definition, Structure, Importance
2. What is Antigen B
     – Definition, Structure, Importance
3. What are the Similarities Between Antigen A and Antigen B
      – Outline of Common Features
4. What is the Difference Between Antigen A and Antigen B
      – Comparison of Key Differences

Key Terms 

ABO Blood Group System, Antigen A, Antigen B, Blood Transfusion

What is Antigen A 

Antigen A is one of the two types of blood group antigens responsible for the determination of blood groups in the ABO blood group system. Generally, it is an oligosaccharide whose chemical structure is GalNAc α1-3 (Fuc α1-2) Gal-. Therefore, the immunodominant sugar of antigen A is GalNAc (N-acetyl-D-galactosamine), which has a  –NHCOCH3 at the C2 position. Furthermore, antigen A occurs in both A and AB blood groups. 

Figure 1: ABO Blood Group System

Moreover, type A blood group contains anti-B antibodies in the plasma. Therefore, blood with antigen B cannot be transfused to an individual with type A blood. However, type AB blood group contains neither anti-B nor anti-A antibodies. Thus, any type of blood of the ABO blood group system can be transfused to an individual with an AB blood group. Therefore, this individual is the universal recipient. In contrast, an individual with an O blood group has both anti-A and anti-B antibodies. It is the most common type of blood group and the universal donor, who can donate blood to any blood group within the same system. Meanwhile, type B blood contains anti-A antibodies, which react with the antigen A in type A blood. Therefore, only type B and type AB blood can be transfused to such an individual.  

What is Antigen B 

Antigen B is the second type of blood group antigen of the ABO blood group system. It is an oligosaccharide. Further, its chemical structure is Gal α1-3 (Fuc α1-2) Gal-. Also, its immunodominant sugar is D-galactose, containing an -OH group at the C2 position. However, antigen B occurs in both type B and type AB blood groups. Moreover, the type B blood group contains anti-A antibodies. 

Similarities Between Antigen A and Antigen B 

• Antigen A and antigen B are the two types of blood group antigens of the ABO blood group system. 
• Both are surface antigens expressed on the surface of the red blood cells. 
• Also, endothelial cells express the corresponding antigens based on the blood group. 
• Besides, both are not protein antigens, but they are oligosaccharides.  
• Importantly, both play a critical role in blood transfusion. 

Difference Between Antigen A and Antigen B 

Definition 

Antigen A refers to one of the two types of blood group antigens especially occurring on red blood cells of A group blood while antigen B refers to the second type of blood group antigens, especially occurring on red blood cells of B group blood. 

Chemical Structure 

The chemical structure of antigen A is GalNAc α1-3 (Fuc α1-2) Gal- while the chemical structure of antigen B is Gal α1-3 (Fuc α1-2) Gal-.  

Immunodominant Sugar 

The immunodominant sugar of antigen A is N-acetyl-D-galactosamine while the immunodominant sugar of antigen B is D-galactose. 

Structural Difference of Immunodominant Sugars 

N-acetyl-D-galactosamine contains -NHCOCH3 at the C2 position while D-galactose contains -OH group at the C2 position. 

Occurrence 

Antigen A occurs in A and AB blood groups while antigen B occurs in B and AB blood groups. 

Antibodies 

Moreover, antigen A is always associated with anti-B antibodies, while antigen B is always associated with anti-A antibodies. 

Reactions with Antibodies in the Plasma 

Furthermore, antigen A reacts with anti-B antibodies in the plasma while antigen B reacts with anti-A antibodies in the plasma. 

Conclusion 

Antigen A is one of the two types of blood group antigens, which determine the blood groups of the ABO blood group system. Significantly, its immunodominant sugar is N-acetyl-D-galactosamine, which contains -NHCOCH3 at the C2 position. Antigen A reacts with the anti-B antibodies of the plasma. Antigen B, on the other hand, is the second type of blood group antigens and its immunodominant sugar is a D-galactose, containing a simple -OH group at the C2 position. It reacts with the anti-A antigens in the plasma. Therefore, the main difference between antigen A and antigen B is their structure, occurrence, and reactivity.   

References:

1. “04. A And B Antigens – All You Need To Know about the ABO Blood Groups (Types).” Google Sites, Available Here.
2. “What Are Blood Group Antigens All About?” Dummies, Available Here.

Image Courtesy:

1. “ABO blood type” By InvictaHOG – Own work (Public Domain) via Commons Wikimedia