Chagigah 21A

האונן ומחוסר כפורים צריכין טבילה לקדש, אבל לא לתרומה.

An acute mourner [onen], i.e., someone who has experienced the loss of a close relative on that day, who had not come into contact with the deceased; and one who is lacking atonement, i.e., someone who still needs to bring an offering to complete his purification procedure, such as a zav or a woman after childbirth, both require immersion in order to eat sacrificial food. The onen would immerse after the day has passed and the one lacking atonement would immerse after the requisite offering is brought. However, immersion in these cases is not necessary for eating teruma.

גמ׳ בקדש מאי טעמא לא? אמר רבי אילא: מפני שכבידו של כלי חוצץ.

Talmud: It is taught in the Mishnah that one may not immerse one vessel inside another if they will be used for sacrificial food, though this may be done for teruma. The Talmud asks: With regard to sacrificial food, what is the reason that one may not immerse vessels in this manner? R' Ila said: Because the weight of the inner vessel causes an interposition between the water and the vessels. That is, the innermost vessel weighs down on the bottom one, not allowing the water to reach the two vessels’ point of contact.

והא מדסיפא משום חציצה, רישא לאו משום חציצה! דקתני סיפא: ולא כמדת הקדש מדת התרומה, שבקדש מתיר ומנגיב ומטביל ואחר כך קושר, ובתרומה קושר ואחר כך מטביל!

The Talmud poses a question on this explanation: But is it not so that since the latter clause of the Mishnah mentions that sacrificial foods and teruma differ with regard to matters of interposition, the first clause of the Mishnah must not be with regard to matters of interposition, but is referring to a different consideration? As it teaches in the latter part of the Mishnah: The characteristics of teruma are not like the characteristics of sacrificial food, as in the case of sacrificial food, if one has a garment or vessel that is tied up he must untie it and dry it, and he may then immerse them, and afterward he may tie them up again if he wishes; but with regard to teruma he may tie up the garment and then immerse it. This latter clause shows that there the rabbis were concerned for any possibility of interposition with regard to sacrificial food, so most likely the earlier clause of the Mishnah is due to a different reason.