So what exactly is the difference between these three antigens? Well, it's basically the absence or presence of an additional sugar molecule. In the case of the O antigen, this is all we have. In the case of the A antigen we have an additional sugar that is linked via the alpha 13 linkage between the Galactose and this and acetyl galactose amine. In the case of the B antigen we have this galactose linked via the alpha one three linkage to another galactose. And that's what differentiates these three types of antigens.