Are antigens glycoproteins?
They can be glycoproteins. But they are just anything "foreign"
When I teach biochem, I talk about carbohydrates - and in that section, we discuss how "whenever there is an interaction between cells (or things interacting with cells), these interactions are almost always mediated by glycosylations". So, if you have E coli in your body, your immune system will interact with its proteins on the surface, of which some are glycosylated, then those antigens are glycoproteins.
Antigens themselves merely induce an immune response. So this can be all kinds of things.