Different county, same state. Can I use MMJ card I have already?

Hello fellow stoners
Here's the deal ; I have just arrived today in San Francisco and I am planning to buy some of that good stuff from the Bay, legally. The thing is I already have a MMJ card issued in Los Angeles county so I was wondering if I could just walk in one of the dispensaries in SF and use this card or do i have to get another one from a licensed doctor here in SF before I can buy anything here?
Pardon my ignorance on this but I never had to buy any weed outside of L.A. so far, my guess would be that I can use the card I already have but I'm not completely sure.
Any helpful answers are welcomed.
Stay High my friends.
:stoned: