Board Thread:Naruto Discussions/@comment-46974839-20200924181420/@comment-46974447-20200924190131

Katsuyu doesn't have any form of "immunity." She is simply highly durable. I would imagine her being cut would work similarly to what she did when she was being constricted by Manda. Or, assuming the blade actually does end up cutting her she could probably just heal herself. But it does also depend on who summons her and what state they are in when they summon her. If Tsunade summons her when she is pretty beaten up and low on chakra, Katsuyu's healing abilities would pale in comparison to when Tsunade is in perfect health with an untouched chakra reserve.

Also, Sakura would never need to hide (but I'll explain that after I get through my current point), but if she were to hide in Katsuyu there would only be one thing able to defeat that defense, and that is literally destroying Katsuyu or forcing her to split in half (which if she were to do this while someone was hiding in her I have no idea how it would work). But, Katsuyu's defense being broken is unlikely (assuming the summoner is at maximum capacity) because she was able to basically tank a large-scale Shinra Tensei. So, yeah, she could hide in her. As for how long, we really have no way of judging that but I would assume for as long as they can keep the summon up. Though, realistically, Sakura wouldn't need to hide. Sakura could defeat Suigetsu without too much difficulty.

I hope this has helped clear things up for you. If you have any more questions just let me know.