You keep your enemies between you and their companion, using them as a shield.

Encounter
Immediate Interrupt      Personal

Trigger: An enemy makes a ranged attack roll against you

Effect: Enemies adjacent to you and in the attack's way grant cover to you against the attack. If the attack misses, the attacker rerolls it against one enemy that granted cover to you against it.

Published in Player's Handbook 3, page(s) 175.