What are the molecules present in antigens of people with blood group a?

Glycoproteins!: Wikipedia actually has a pretty good article on this: https://en.wikipedia.org/wiki/ABO_blood_group_system A, B, AB, and O cells all begin the same, with the production of a molecule on the RBC (red blood cell) surface called H antigen. The genes coding for A or B blood group (or AB, if you have one of each) are actually just chemical modifications of this common H antigen. Pretty cool!