Waldeyers Ring..............Google it.
The tonsils backed up by the adenoids and other lymph tissue in the back of the mouth, from a formidable barrier to the entry of infection.
Not the only barrier to infection.
Sometimes due to repeated infections, the tonsils fail to provide the defence mechanism and become the focus of infection themselves and need to be removed.
The adenoids on the other hand, rarely, if ever become infected, but unfortunately may block the Eustachian Tube, contributing to the common childhood disease known as "Glue Ear" and removable becomes a necessity.