As your foe unleashes its attack, you realize that you have read about this sort of danger and shout a warning to your friends.

Encounter
Immediate Interrupt      Personal

Trigger: A creature you can see makes an attack roll for an area or a close attack that includes you as a target

Effect: You and each ally included as a target of the triggering attack gain a +2 power bonus to all defenses against it.

Published in Player's Handbook 3, page(s) 165.