Actually Immunet (Free) uses their own engines (in the cloud), these also have the signatures from ClamAV in the cloud. (Paid uses Bitdefender locally and Immunet cloud)
But you can choose to enable the Clam engine locally on your computer if you choose to, this will raise the protection somewhat I was told by Alfred, one of the founders of Immunet.
I hope this can clarify a bit.
/W