Numbers 18:3 "They shall not approach so that they shall not die, neither they nor you"

Both, Priests and Levites serve in the Beit HaMikdash.

There are specific duties that are entrusted to the priests and other duties entrusted to the Levites. The priests are forbidden from doing any service assigned to the Levites; and likewise the Levites are forbidden from doing any service assigned to the priests.