720 ILCS 5/7-6

Private person's use of force in making arrest.

(a) A private person who makes, or assists another private person in making a lawful arrest is justified in the use of any force which he would be justified in using if he were summoned or directed by a peace officer to make such arrest, except that he is justified in the use of force likely to cause death or great bodily harm only when he reasonably believes that such force is necessary to prevent death or great bodily harm to himself or another.

(b) A private person who is summoned or directed by a peace officer to assist in making an arrest which is unlawful, is justified in the use of any force which he would be justified in using if the arrest were lawful, unless he knows that the arrest is unlawful.