Can a grabbed creature stand up from prone?

Question

One of my players, a thri-kreen berserker, uses Run Down and World Serpent's Grasp to prone enemies. He then used his action point to make a grab attack, with the intent of holding the enemy to the ground.

Grab says it immobilizes the target. I allowed the grab to prevent the target from standing because it made sense. Can anyone explain why this wouldn't work?

Answer 1

Grabbing someone does NOT stop them from standing up from prone.

From PHB1, pg290, describing the grab action:

Hit: The enemy is immobilized until it escapes or you end the grab.

Grab inflicts immobilize, nothing else (barring some brawler fighter feats).

From PHB1, pg277:

Immobilized: You can't move from your space, although you can teleport and can be forced to move by a pull, a push, or a slide.

While standing up is a move action, it does not entail leaving your space, so being grabbed/immobilized does not stop you from standing up.

There's a fighter feat, Pin Down (Dragon 368), that specifically allows this:

Benefit: A prone target you are grabbing can't stand until you end the grab or until it escapes.