please some help

Hi,
I have a table called clients, there are two fields inside this table, one called comments and one called History, both fields are text area type.
What I'm trying to do is the following

1- Users can type whatever they want in the comments field.

2- When comments filed updated by another, I need the newly updated text to be inserted into the History field, the history filed has to be read-only for all users.

The main idea is to control permissions on a text area field so users can update the text area field but they can't change the old updates. It’s kind of protected History.
regards