Pesachim, Chapter 7, Mishnah 5. This mishnah deals with the law of tumah that befell the meat of the Pesach offering or its fats before the blood was sprinkled, and with the difference between the Pesach offering and all other offerings in this regard.
"Nitma habasar v'chelev kayam - eino zorek es hadam" - if the meat became tamei and the fats remain intact, he does not sprinkle the blood:
The case here is one in which all of Israel is tahor, but the meat of a particular Pesach offering became tamei, while the fats, the portions burned on the altar, remain intact and did not become tamei. Since the meat is no longer fit to be eaten, there is no purpose in sprinkling the blood, for there is nothing to be gained by bringing a Pesach offering that cannot be eaten, and the Pesach offering will not be valid.
"Nitma hachelev uvasar kayam - zorek es hadam" - if the fats became tamei and the meat remains intact, he sprinkles the blood:
In the reverse case, where the fats that are to be burned on the altar became tamei while the meat of the Pesach remains in its state of taharah, the blood is sprinkled, because there is a purpose in doing so: the meat is fit to be eaten. Burning the fats is indeed part of the mitzvah, but it is not indispensable and it does not affect the validity of the Pesach offering. Therefore, even though there is no way to burn them on the altar because of their tumah, and they are burned outside in the place where disqualified consecrated items are burned, this is of no consequence - the main point is that the Pesach offering be fit to be eaten.
"Uvamukdashin eino kein" - and with consecrated offerings it is not so:
With the other offerings the law is not the same; rather, "af al pi shenitma habasar v'chelev kayam - zorek es hadam" - even though the meat became tamei and the fats remain intact, he sprinkles the blood. With other offerings, if the meat that is eaten by the kohanim became tamei and is no longer available, while the fats that are burned on the altar remain intact, that alone is sufficient reason to sprinkle the blood, in order to make it possible to burn the fats on the altar.
It is worth noting that the Rambam writes that the reverse is also true: if the fats of the other offerings became tamei while the meat is tahor and fit for the kohanim to eat, this too is a valid reason to sprinkle the blood.
The principle that emerges is this: neither of these two items affects the validity of the offering, yet each of them has a function. If he sprinkles the blood, he will be able to eat the meat or, alternatively, to burn the emurim; and if he does not sprinkle the blood, he will not be able to do either of them. Therefore each one on its own constitutes a valid reason to continue the process of offering that sacrifice.
In summary: with the Pesach offering, everything depends on its fitness to be eaten - if the meat became tamei, the blood is not sprinkled; if only the fats became tamei, the blood is sprinkled. With the other consecrated offerings, by contrast, the presence of either one of the two, the meat for the kohanim to eat or the fats to be burned, is enough for there to be a purpose in sprinkling the blood and in continuing the service of the offering.