Question: Why is the blood of the lamb (and not the blood of any other animal) used to make the mark on the door posts of Jewish homes for the angel of death to pass over in the story of the Exodus?

Answer: The smearing of the blood on the door posts was done one time in history when the Jews were still in Egypt. The blood was taken from the sacrifice of the Paschal Lamb. One of the reasons given for why a lamb was used is that since the Egyptians worshiped the lamb as part of their pantheon, the Jews were instructed to make the sacrifice specifically from that animal. It was to be a sign to the Egyptians – and to the Jews – that the Jews were completely free of the Egyptian culture of which the lamb played a central role.

Rabbi Moshe Rosenstein