A creature can try to protect an ally against an enemy. This action represents a parry, a shield block, or some other action that makes it harder for the enemy to hit the ally.
Aid an Ally’s Defenses
Action: Standard action. When a creature takes this action, it chooses an enemy adjacent to it.
Grant Bonus to Defenses: The creature chooses an ally. That ally gains a +2 bonus to all defenses against the chosen enemy’s next attack against it. This bonus ends if it is not used by the end of the aiding creature’s next turn. A creature can take the aid defense action only once to affect a particular ally’s defenses. However, up to four creatures can take the action to affect the same ally’s defenses.
Published in Rules Compendium, page(s) 238.