1. Blood group tupe A antigen is a complex oligosaccharide which differs from H antigen present in type O individual by the presence of terminal
(1) Glucose          (2) Galactose
(3) Fucose           (4) N-acetyl galactosamine

Detailed explanation

• The basic structure on RBCs is the H antigen, a short oligosaccharide chain. In type O individuals, this H antigen remains unmodified, so their red cells express only H. In type A individuals, a specific glycosyltransferase adds an extra sugar, converting H into the A antigen. This added terminal sugar is N-acetyl galactosamine.

Option-by-option:

1. Glucose

• Glucose is a common monosaccharide but is not the distinguishing terminal residue that converts H antigen to A antigen.

2. Galactose

• Galactose is the immunodominant sugar for the B antigen, not the A antigen. The B-transferase adds galactose to the H antigen to form the B determinant.

3. Fucose

• Fucose is already part of the H antigen core (α1,2-linked to galactose). It is not the additional terminal sugar that differentiates group A from group O.

4. N-acetyl galactosamine – correct

• The A-transferase adds N-acetyl galactosamine to the terminal galactose of the H antigen, creating the A-specific oligosaccharide. This is the key structural difference between the A antigen and the unmodified H antigen of type O red cells.

Thus, blood group A antigen differs from the H antigen of type O individuals by the presence of a terminal N-acetyl galactosamine.