הָאוֹנֵן וּמְחוּסַּר כִּפּוּרִים צְרִיכִין טְבִילָה לַקֹּדֶשׁ אֲבָל לֹא לַתְּרוּמָה: 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.
גְּמָ׳ בַּקֹּדֶשׁ מַאי טַעְמָא לָא אָמַר רַבִּי אִילָא מִפְּנֵי שֶׁכְּבֵידוֹ שֶׁל כְּלִי חוֹצֵץ GEMARA: It is taught in the mishna 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 Gemara asks: With regard to sacrificial food, what is the reason that one may not immerse vessels in this manner? Rabbi 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.
וְהָא מִדְּסֵיפָא מִשּׁוּם חֲצִיצָה רֵישָׁא לָאו מִשּׁוּם חֲצִיצָה דְּקָתָנֵי סֵיפָא וְלֹא כְּמִדַּת הַקֹּדֶשׁ מִדַּת הַתְּרוּמָה שֶׁבַּקֹּדֶשׁ מַתִּיר וּמְנַגֵּיב וּמַטְבִּיל וְאַחַר כָּךְ קוֹשֵׁר וּבַתְּרוּמָה קוֹשֵׁר וְאַחַר כָּךְ מַטְבִּיל The Gemara poses a question on this explanation: But is it not so that since the latter clause of the mishna mentions that sacrificial foods and teruma differ with regard to matters of interposition, the first clause of the mishna 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 mishna: 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 Sages were concerned for any possibility of interposition with regard to sacrificial food, so most likely the earlier clause of the mishna is due to a different reason.
רֵישָׁא וְסֵיפָא מִשּׁוּם חֲצִיצָה וּצְרִיכָא דְּאִי אַשְׁמְעִינַן רֵישָׁא הֲוָה אָמֵינָא הַיְינוּ טַעְמָא דִּלְקֹדֶשׁ לָא מִשּׁוּם כְּבֵידוֹ שֶׁל כְּלִי דְּאִיכָּא אֲבָל סֵיפָא דְּלֵיכָּא כְּבֵידוֹ שֶׁל כְּלִי אֵימָא לְקֹדֶשׁ נָמֵי לָא הָוֵי חֲצִיצָה וְאִי אַשְׁמְעִינַן סֵיפָא הֲוָה אָמֵינָא הַיְינוּ טַעְמָא דִּלְקֹדֶשׁ לָא מִשּׁוּם The Gemara answers: No, this is not necessarily so. In both the first clause and the latter clause of the mishna the concern is due to interposition, and it is nevertheless necessary to teach us both cases. For had the mishna taught us only the first clause, i.e., that one may not immerse one vessel within another, I would have said that this is the reason one may not do so for sacrificial food: Because there is the weight of the interior vessel that must be taken into account. But in the latter clause, where there is no weight of a vessel to be considered, I will say that it is not considered an interposition for sacrificial food either. And had it taught us only the latter clause dealing with the knotted garments, I would have said that this is the reason one may not do so for sacrificial food: Because it is possible