Parah, Chapter 11, Mishnah 5. Our chapter is occupied with the purity and impurity that surround the mei chatas, the waters of the parah adumah, and along the way it takes up the status of various people who are obligated to immerse in a mikveh. The previous mishnah dealt with someone whose immersion is required by Torah law. Now the Mishnah turns to a lighter case: a person whose obligation to immerse comes only from the words of the Sages.
Here is how the Mishnah puts it. "Kol hataun bias mayim", whoever is required to enter the water, "midivrei sofrim", on the authority of the Sages and not because the Torah itself demanded it. Such a person "metamei es hakodesh": consecrated food that he touches becomes impure. "Ufosel es haterumah": terumah in his hands is rendered unfit. Yet "umutar bechulin uvama'aser", ordinary food and ma'aser remain open to him, "divrei Rebbi Meir", and this ruling belongs to Rebbi Meir. Who falls into this category? Take someone who swallowed impure food or impure drink. His tumah rests on rabbinic authority alone, and so does the immersion he now owes.
The Mishnah then records a dissenting view: "Vachachamim osrin bama'aser." The Chachamim forbid him in ma'aser, meaning ma'aser sheni. In their opinion the leniency extends only as far as chullin; ma'aser sheni, which carries a measure of sanctity, is closed to him until he immerses. The halachah follows the Chachamim: such a person is forbidden in ma'aser, and permitted in everything else on the list.
Next the Mishnah describes his status once he has immersed: "Le'achar biaso mutar bechulan." After his immersion he is permitted in all of them. Note that this is so even though the sun has not yet set on the day of his immersion. Where the impurity is only rabbinic, immersion alone completes his purification, and he need not wait for nightfall in order to eat any of these foods.
The Mishnah closes with the question of walking into the Mikdash. "Ve'im ba el hamikdash", should he in fact enter the sanctuary, then "bein lifnei biaso", whether he did so before going down to the water, "uvein le'achar biaso", or only afterwards, in either case he is "patur", free of any liability. His tumah was never imposed by the Torah but by the Sages, so no punishment follows his entry. Scripture's warning against approaching the sanctuary in a state of impurity was aimed at a person who became tamei through an av hatumah on the Biblical level, and that description simply does not fit him.
The mishnah thus draws a clean line. Rabbinic impurity is real enough to bar a person from kodesh, from terumah and (according to the accepted view) from ma'aser sheni, yet it is light enough that a simple immersion restores him at once, and light enough that it does not render him liable for entering the Beis Hamikdash.