App sandboxing isolates apps from the critical system components of your Mac, your data and your other apps. Even if an app is compromised by malicious software, sandboxing automatically blocks it to keep your computer and your information safe.And Gatekeeper makes it safer to download apps by protecting you from inadvertently installing malicious software on your Mac. So we now know that sandboxing and Gatekeeper will protect you from any nasties. Phew. But hang on http://techleaks.us/apple-missed-iworm-malware-in-security-check/ . If you download a malicious app via uTorrent (for example), Gatekeeper won't bat an eyelid. If you try to install an app infected by iWorm, XProtect will block the install but it won't go looking to see if you are already infected with it. And sandboxing makes no difference as it is limited to certain apps/plugins. Apple security- the gift that keeps giving.