Pesachim Chapter 7, Mishnah 7. The Torah teaches that the tzitz, the headplate worn by the Kohen Gadol on his forehead, serves to effect acceptance - to render valid offerings that would otherwise be disqualified on account of tumah. This is the function the Torah assigned to the tzitz, and in our mishnah we will learn its scope and its limits.
"HaPesach shenizrak damo ve'achar kach noda shehu tamei" - a pesach offering whose blood was sprinkled, and afterward it became known that it was tamei:
If the blood of a pesach offering was sprinkled on the altar, and only afterward it became known that it was tamei - whether its meat had become tamei or its blood had become tamei - "hatzitz meratzeh" - the tzitz effects acceptance. The offering is valid, and the owners are not required to bring a second Pesach. Even so, the pesach is not eaten, since eating an offering in a state of tumah was permitted only when the entire community is tamei; the offering itself, however, is valid. This is what the tzitz accomplishes.
"Nitma haguf - ein hatzitz meratzeh" - if the body became tamei, the tzitz does not effect acceptance:
But if the person himself became tamei, the tzitz does not effect acceptance and its power avails nothing, and he must bring a second Pesach. The reason lies in what the Sages said regarding these two offerings - the nazir and the one who brings the pesach, that is, a nazir bringing the offerings of his nezirus and one bringing the pesach offering:
"Hatzitz meratzeh al tum'as hadam" - the tzitz renders the offering valid when the blood became tamei, and all the more so when the meat or the fats (the eimurim) became tamei.
"Ve'ein hatzitz meratzeh al tum'as haguf" - when the person himself is tamei, the tzitz does not confer validity upon the offering.
"Nitma tum'as hatehom" - if he contracted tumas hatehom:
To this rule there is an exception: tumas hatehom, "tumah of the depths." This refers to a person who contracted the tumah of a corpse from a body that was hidden away and that no one in the world knew about, and only afterward was discovered. In such a case, if the nazir or the one bringing the pesach offering discovers, after the offering was brought, that he had been tamei with a tumah of the depths that no one knew about - "hatzitz meratzeh" - the tzitz effects acceptance. It is a halachah given to Moshe at Sinai that whenever no one knew of this tumah and it came to light only afterward, his pesach is valid, the offerings of his nezirus are valid, and he is not required to bring other offerings.
In summary: in this mishnah we learned the parameters of the acceptance effected by the tzitz - regarding a pesach whose blood was sprinkled and only afterward it became known that it was tamei, the tzitz effects acceptance and the offering is valid, though it is not eaten; regarding tumah of the body the tzitz does not effect acceptance, both for a nazir and for one bringing the pesach, and he must bring a second Pesach; while regarding tumas hatehom, it is a halachah given to Moshe at Sinai that the tzitz effects acceptance and the offering is valid.