Aid Of Opportunity

During a combat encounter, you can spend an immediate action in order to call for help; upon doing so, any ally that is adjacent to you can choose to expend an attack of opportunity in order to perform the aid another action on you. For example, Malcolm the wizard and Deneb the dauntless defender are fighting an Ice Troll. The Ice Troll makes an attack against Malcolm, who calls for help from Deneb; Deneb provides Malcolm with an aid of opportunity and his aid another check is successful. As a result, he increases Malcolm's AC by 2, which causes the Ice Troll's attack to miss, since it had rolled a 16 and Deneb's aid of opportunity increased Malcolm's AC from 15 to 17.