Which statement is true concerning the I antigen?

• A. It is well developed on cord red blood cells.
• B. It is poorly developed on cord red blood cells.
• C. It is absent on cord red blood cells.
• D. It is identical to the adult I antigen.

Answer: (B)

I antigen expression on red blood cells changes with age. In cord blood, the surface carbohydrate chains are predominantly of the i form, and the I antigen is not yet well formed. As a person ages, i is progressively converted to I, so the adult pattern shows robust I antigen expression.

That’s why the statement about I antigen being poorly developed on cord red blood cells is the best choice. It isn’t fully absent, but it is not yet well formed in newborns. It also isn’t identical to the adult I antigen, since the cord pattern reflects the immature state. The idea that it is well developed contradicts the neonatal pattern.