Using expansion packs

[Munchkin JK]

That's right, it's time for another newbie question! Fun, right?

So, is there any kind of official rule dealing with expansion packs? More specifically, I bought Munchkin 2: Unnatural Axe and was wondering if I have to use all the cards in the pack or can I just use the Orc race cards (and associated items that Orcs can use) without using all the other cards? Or are you required to incorporate all of the new cards?

Thanks, as always.