Thanks Rajeev, this worked for me.
I'm not sure why MS decided to put the Windows Defender controls under Action Centre and not under ... you know ... Windows Defender. Seems like a stupid thing to do. It's not intuitive and that's a major issue for GUI's.
It's just like trying to find the shutdown button on the original release of Windows 8 (using a mouse and keyboard). If you have to Google (or Bing *cough*) for the simple things, then there is something wrong with the GUI.