I know I'm probably bugging you guys, but:

What system protects the body from foriegn substances?

It has 6 letters, and the 4th letter is a "u" and the 6th letter is a "t".

imune system

wait...its actually spelled immune system

No, but thanks! Unfortunately, it's only 6 letters long.

This is what I have of it so far:

_ _ _ u _ t