(Mishnah): No one may enter the Azarah for Avodah, even if he is Tahor, until he immerses.

[The Kohen Gadol] immerses five times and is Mekadesh [his hands and feet] 10 times in the day [of Yom Kipur]. All of them are in the Kodesh, except for this [the first one before entering the Mikdash].

(Gemara): Our Mishnah means that no one may enter the Azarah or for Avodah, even if he is Tahor, until he immerses. It is not only for Avodah, rather, even not for Avodah.

Question (to Ben Zoma): What is the reason for this Tevilah?

Answer (Ben Zoma): If when [the Kohen Gadol] enters from Kodesh [the Azarah] to the Kodesh [ha'Kodoshim], he must immerse, from Chol to Kodesh, all the more so!

Question: Does he say that one who enters from Kodesh to Kodesh needs Tevilah [due to a stringency of Kodesh, all the more so one who enters from Chol to Kodesh? (ALEI TAMAR)

Answer (R. Shamai): (No. Rather,) he means if from Kodesh to Kodesh, for which there is no Chiyuv Kares (if he became Tamei in the Azarah and left via the Heichal), he must immerse, from Chol to Kodesh, for which there is Kares (if he became Tamei in the Chol and entered the Azarah, the all the more so he must immerse!

Question: Even from Kodesh to Kodesh, there should be Kares, like a Mishnah teaches there "if he [became Tamei in the Azarah, and] bowed or delayed enough to bow [he is Chayav Kares]'';

Question: What is the Shi'ur "enough to bow''?

Answer #1 (R. Simai citing R. Yehoshua ben Levi): It is enough to say "va'Yichre'u Apayim Artzah Al ha'Ritzpah va'Yishtachavu.''

Answer #2: R. Avahu adds [that he must say also] until "v'Hodos la'Shem Ki Tov.''

Answer #3: R. Mana adds until "Ki l'Olam Chasdo.''

(Beraisa - R. Yehudah): They enacted this Tevilah only due to Tevilas Serach. [I.e.] sometimes one goes to immerse [before entering the Azarah], and he remembers an old emission, and he refrains and goes away. (Even after Tevilah, he may not enter the Azarah until Ha'arev Shemesh.)