Forum:Conflicting monster effect negating effects

What I want to know is what happens if two monsters both negate each other's effects. Say I had "Obilisk the tormentor" battle "sleeping giant thud. Thud's effect says it's immune to monster effects, but obilisk negates all monster effects the turn it is summoned. My question is would they both negate each other's efffects? Or is there some rule for these kinds of situations