Board Thread:Versus Debates/@comment-9782468-20160602162113/@comment-9782468-20160604060832

Sharingan users as a whole usually can notice a Genjutsu being cast because of the Sharingan, so the fact that she managed to get the illusion to activate the jutsu on Itachi in the first place is pretty good to me.

Take this example, much of Konoha fell under Kabuto's genjutsu but some skilled Shinobi noticed and dispelled it before they were affected. Basically Itachi should be skilled enough to notice and dispell any Genjutsu before it's being cast as long as it's within his skill to do so. So Kurenai managed to at least begin illusions on him.

Itachi complimented her skill too