The Huntress Agent itself will run alongside any other security products (sometimes you may need to whitelist Huntress).
However, you cannot utilize our Managed Microsoft Defender AV features.
Microsoft Defender disables itself when another AV is installed to prevent conflicts. Microsoft Defender has some functions that you can re-enable, but it won't be fully functional. All 3rd party AVs need to be uninstalled for Microsoft Defender to work at full capacity.