Antigenic Determinants in ABO Blood System
The ABO blood group system is based on the presence or absence of specific antigens on the surface of red blood cells. These antigens determine an individual's blood type (A, B, AB, or O).
Nature of ABO Antigens
The key molecules responsible for the antigenic properties in the ABO system are located on the surface of red blood cells. These are complex molecules, but the specific structures that the immune system recognizes as 'A' or 'B' are terminal sugars (carbohydrates).
- Antigen A is characterized by the presence of N-acetylgalactosamine.
- Antigen B is characterized by the presence of galactose.
- Individuals with type O blood lack both A and B antigens but have a precursor structure (H antigen).
These carbohydrate structures are typically attached to either lipid molecules (forming glycolipids) or protein molecules (forming glycoproteins) embedded in the red blood cell membrane. However, the specific sugar molecule itself is the primary determinant of the antigen's identity.
Why Carbohydrates?
The structural variations in the carbohydrate chains are what differentiate the A, B, and H antigens. Antibodies in the blood will react specifically with these carbohydrate structures. Therefore, the carbohydrate portion acts as the antigenic determinant.
Evaluating Other Options
- Nucleic acids (like DNA or RNA) are not the primary antigens in the ABO system.
- Lipids can be part of the molecule (glycolipids), but they are not the specific antigenic determinants themselves; the attached carbohydrate is.
- Proteins can also be part of the molecule (glycoproteins), but similar to lipids, the specific carbohydrate structure determines the A or B specificity, not the protein backbone.
Thus, the antigenic determinants in the ABO blood group system are fundamentally carbohydrate in nature.
Important Questions from Structure and Function of Biomolecules
- Match the carbohydrates in Column I with the correct glycosidic linkages involved in Column II.
Column I Column II P. Amylose 1. $\beta$ ($1\to 4$) Q. Sucrose 2. $\alpha$ ($1\to 4$) R. Amylopectin 3. $\alpha 1\to \beta 2$ S. Cellulose 4. $\alpha$ ($1\to 4$), $\alpha$ ($1\to 6$) - Match the biomolecules in Column I with their function given in Column II
Column I Column II P. Lectins 1. Acts as ATP dependent pump to efflux out small molecules Q. P-glycoprotein 2. Targets lysosomal enzymes to their destination R. Digitoxigenin 3. Specific carbohydrate binding proteins S. Mannose 6-phosphate 4. Inhibits Na$^+$-K$^+$ pump - Which one of the following coenzymes is utilised by alanine racemase for the conversion of L-Alanine to D-Alanine?
- Correctly match the following Monosaccharides with their respective Epimers.
Monosaccharide Epimer P. D-mannose 1. C-3 epimer of D-glucose Q. D-allose 2. C-4 epimer of D-glucose R. D-galactose 3. C-4 epimer of D-mannose S. D-talose 4. C-2 epimer of D-glucose 5. C-5 epimer of D-glucose - Which of the following show(s) optical activity at 100 mM concentration in water?
