Originally posted by: Budweineken
I'm pretty sure that Shizune is also one of the special cases....

I really don't know for how long she has been traveling with Tsunade... but I think that she began before she became a juunin or a chuunin for that matter...

Still she's considered good enough to handle juunin class missions...

I may be totally off... but whatever...
I think the rank system exists only in the various ninja villages as a form of classification of work and missions.
Since Shizune wasn't in Konoha village or any other village for that matter, rank didn't matter to her. But of course, she is a jounin-level ninja dedicated to the protection of Tsunade