Minions and turrets will only deviate from their standard behavior to come to the aid of an ally under duress. Whenever any unit is under attack, it generates a ‘call for help’. Minions and turrets have slightly different reactions to a call for help.
Under normal circumstances, a minion’s behavior is extremely simple. Minions follow their attack route until they reach an enemy to engage. Every few seconds, they will scan the area around them for the highest priority target.
When a minion receives a call for help from an ally, it will evaluate its current target in relation to the target designated by the call. It will switch its attack to the new target if and only if the new target is of a higher priority than their current target.
Minions prioritize targets in the following order:
Note: Single-target spells such as Annie's Disintegrate or Kassadin's Null Sphere will not generate a call for help.
Because they are continuously reevaluating their target, minions will lose interest in an enemy champion if that champion breaks off an engagement with them, provided that there is any other target left for them to choose within range.
Remember, however, that minions will only reevaluate their target every few seconds. This means that if your champion initiates an attack against an enemy champion, your champion will continue to be viewed as the most threatening target for a short time after you have broken off the attack. This is true even if one of your allied champions has joined the fight, since minions always prioritize their current target over a new target of equal priority.
A turret prioritizes targets somewhat differently than a minion. Its targeting priorities are much more streamlined, and unlike a minion, a turret will never lose interest in your champion as a target. If your champion becomes the target of a turret, your only recourse is either to flee out of its range, or to destroy it.
A turret prioritizes targets in the following order:
Note: Any time a champion takes damage near a friendly turret, it will immediately try to attack the aggressor unless otherwise occupied with an enemy champion.
Tip: If you are going to pick a fight by one of your opponent’s turrets, make sure your most durable champion scores the first hit. This will draw the turret’s fire away from the more fragile members of your team.