Although Microsoft provides Windows Defender as an integrated part of Windows 10 they do not mandate that as your only option.
Once you install your favorite third party anti-virus/malware solution Windows Defender gets out of the way and is turned off so it does not cause any conflicts with your scanning/protection software of choice.
In the recently released Windows 10 Anniversary Update there is a new option for those of you using third party security software that allows you to take advantage of the years worth of research Microsoft has invested into protecting systems from viruses and malware.
Source : SuperSite for Windows
