what is the RBC antigens?

Answer 1

The major red blood cell antigens are from the "A B O" system.
The Rhesus antigens are the next most important with phenotypes consisting of combinations of C, c, D, E, and e antigens.
There are also plenty more groups of antigens on red blood cells like Kell, Duffy, and Lewis.
Their presence is detected by agglutination tests in transfusion laboratories to select compatible blood. The "A B O" and Rh systems can cause acute haemolytic transfusion reactions if miss-matched, but the presence of the others are also looked for because they too can cause problems in the recipient.

As a broad non-specific definition: they are epitopes on red blood cells that can be recognised by the immune system and have antibodies made to target them.

Answer 2

The RBC Ag is nothing but your blood group Ag.the four blood groups classified as ABO grouping is A,B,AB,O.The A blood group individuals have on their RBC the A Ag and ANTIb Antibody and the B blood group has B Ag and ANTI A antibody and AB has both the Ag and no antibody and O has both antibody and no Ag . so when blood transfusion occurs the Ag in the RBC can react with the antibody of the donor and cause serious agglutination reactions

note:Ag means antigen