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When enabled, Windows 10 will use the Windows Defender scanning engine to periodically scan your PC for threats and remediate them. You can choose to enable an optional, limited protection feature, called limited periodic scanning. On endpoints and devices that are protected with a non-Microsoft antivirus or antimalware app, Windows Defender Antivirus will automatically disable itself. You should leave everything as it is, leave Bitdefender installed and working, while Windows Defender will not intrude. It is not easy to uninstall Defender and Microsoft made sure to allow you to use other antivirus without issues. Short answer Defender is automatically suspended when 3rd party AV is installed.
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