Which antigen is associated with phenotype 'A' in the ABO system?

The association of the 'A' phenotype in the ABO blood group system is characterized by the presence of the A antigen on the surface of red blood cells. Individuals with this phenotype will express the A antigen due to the specific gene that encodes for the enzymatic modification of the carbohydrate structures on the blood cells.

In addition to having the A antigen, these individuals typically produce anti-B antibodies in their plasma, which are directed against the B antigen. This is a key aspect of how blood group compatibility is determined, as the presence of these antibodies means that if they were exposed to type B blood (which contains the B antigen), it would trigger an immune response.

The correct answer highlights this unique combination of having the A antigen while also possessing the anti-B antibodies, which is essential for understanding blood typing and transfusion compatibility. This understanding is crucial in clinical settings to prevent transfusion reactions.