ASP.NET: How to detect lost focus in user control

Is there a way to detect lost focus of a user control? I have 5 user controls in an AJAX Tab Container. One for each tab panel. I want to save the data of each panel as soon as it looses focus. My workaround so far is to catch the ActiveTabChange event and the Click events of the buttons that are on each panel for extra navigation forward and backwars in the tabs. I would like to catch the loosing of the focus on each user control, e.g. if another programm is selected, or a click on any other element on the parent page is done. So in general who can i detect when a user control loses focus?

Marcus