Immune: characters with the Uncanny Dodge class feature
A character who has not yet acted during a combat is flat-footed, unable to react normally to the situation. A flat-footed character loses his Dexterity bonus to AC (if any) and cannot make attacks of opportunity.
Marius, Scholai Armsmaster Says: Because being flat-footed removes a creature’s dexterity bonus to AC, it makes them vulnerable to sneak attacks. One commonly overlooked advantage to winning initiative is that your enemies cannot take attacks of opportunity. This allows you to dart to the perfect position in the battlefield. For melee fighters, this is usually next to an archer or wizard.