How Do Antigens Get Into the Urine?

The sea polysacaride is getting filtered into the urin, and then it's just staying in there. So how does i guess kunt of like that brings us two questions. The first is, how does the anogen get into the urine if the numa cacus is not in the urinary system? And presumably that's by the blood, but then why don't we just measure it in the blood?