Kakuzu has an interesting body structure as many of you have already noticed, Kakuzu is able to deploy hearts using his tendrils (he's a rag doll after all) which are known as "ghosts" or masks. Now his body structure is the key to countering ANY, and ALL Genjutsu-based approaches, and here is why.
Take a look at this quote:
The only known organ within Kakuzu's body are hearts (need to use various elemental nature affinities) however, some may argue that it is irregular that he doesn't possess a brain, and that he more than likely possesses one. I say this is false due to that fact that we've seen another Akatsuki member with a "5" in intelligence, along with no bodily functions limited; that shinobi would be Sasori.
You must be registered for see images
You must be registered for see images
The only living things on Sasori is his core, and what exactly did his core contain? Well it's rather interesting:
So due to the quote above, all that is needed to operate the shinobi body is a chakra storage in which is capable of executing chakra through out their bodies to perform techniques, and bodily movements; basically chakra controls everything. Kakuzu has a similar structure to Sasori, Kakuzu is a rag doll, while Sasori is a puppet they're both entities that we don't believe to have brains correct? (a puppet, or a doll having a brain would be quite absurd).
You must be registered for see images
You must be registered for see images
His hearts channel chakra throughout his whole body enabling him to control his tendrils (which take chakra to use as well; Jiongu), along with his elemental ninjutsu-based assaults/defenses. This is how his masks were able to fight independently even when he died multiple times, it's due to the fact that "the heart serves as a chakra executor" (all the body needs to function in the NarutoVerse is a chakra storage in which transfers chakra through the body). Here you can see a dead Kakuzu, and his mask (containing each a single heart) acting dependent sort of like a symbiotic attachment like Kisame, and Samehada's relationship:
You must be registered for see images
To furthermore prove that Kakuzu is truly immune to Genjutsu, take a look at this quote about Kinjutsu (Jiongu is a Kinjutsu after all):
Pay attention heavily to the second category, "violates the laws of nature". Well isn't that exactly what Jiongu does? It allows the user a state of immortality, by the integration of a maximum of 5 hearts, while also allowing the user to take on a rag doll like body structure completely being made up of threads/tendrils. Thus I can conclude that Kakuzu does not have a brain, uses his heart as a chakra executor to transfer chakra into his tendrils to perform bodily movements along with the ability to perform elemental attacks, and grants him immunity to Genjutsu.