I Have A White Line On The Back Of My

I have a white line on the back of my throat and it runs up as far as I can see and down as far as I can see. I just recently went to the dentist about some blisters I had in my mouth and he said it was a viral infection, but I didn't pay attention to the white line I had in the back of my throat until tonight and I don't know if it was there before or not. What could that be?