how should you deal with somebody who says you're 'not' something that you are

like i am of italian descent but i am british and i once had someone try to tell me that i am 'not' of italian descent, which i found bizzare and i was in disbelief about, because i know that i am.

anyway, how do you deal with a situation like that? someone who is obviously trying to get a reaction from you by dicatating to you what you are and what you are not? unbeilvable.