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.
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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).
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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:
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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.
