In general, a Cohen / כהן, a descendant of Aaron the High Priest, may not come in contact with any dead body. Additionally, a Cohen may not enter a building where a dead body of a Jew lays. Outdoors, a Cohen is forbidden to come within Arba Amot / ארבע אמות — four cubits of a Jewish corpse or a Jewish grave. A Cohen is commanded (Leviticus 21:1-2) to be in a state of purity and avoid ritual defilement by a corpse, which is ritually unclean. Therefore, Cohanim / כהנים - the priests, usually do not participate in the burial unless it is one of the closest relatives mentioned below.
A Cohen is obligated to ritually defile himself and bury a Met Mitzvah / מת מצוה — a deceased who has nobody else to take care of his or her burial. A Cohen is also obligated to ritually defile himself in case of the following seven closest relatives: father, mother, wife, son, daughter, paternal brother and paternal unmarried sister. He may come in contact with those seven closest deceased relatives and ritually defile himself even if there are other Jews who are not Cohanim and who are ready and willing to take care of the burial. Maternal brothers or sisters or any sisters who are or have been married are not included in this category, even though a Cohen is obligated to mourn for them. The Cohen’s obligation to bury his dead extends until the time that the grave is covered. At that point the Cohen must leave the Jewish cemetery.
A Cohen may not enter a funeral home when dead bodies are inside, unless the remains of one of his closest relatives mentioned above are there as well. It is the custom of Cohanim, who go to funerals to show respect for the deceased, to stand outside of the funeral home until the deceased is brought out to be buried. The Cohanim then follow at a respectful distance to the cemetery gates and stop there; they do not enter the Jewish cemetery.
If the deceased was from a family of Cohanim, he will usually be buried in the “Cohanim section” which is located adjacent to the outer edge of the Jewish cemetery, in order to enable his relatives who are Cohanim to attend the burial as well as family memorials while standing outside the cemetery boundaries. Otherwise, a Cohen is not allowed to attend unveilings of even his closest relatives, since it is forbidden for him to enter the cemetery.
A descendant of Cohanim whose priestly status is lost or questionable, e.g., a Mamzer / ממזר or a Chalal / חלל, should ask a qualified rabbi about his obligations and status.