Why is blood needed to sign a summoning contract and to use the summoning jutsu?

In Naruto, I have seen that to create summoning contract and to use summoning jutsu, the user uses their own blood. Why is this required? What is the significance of using blood here?

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2 Answers 2

You can read in the Wikia that

The Summoning Technique is a space–time ninjutsu that allows the summoner to transport animals or people across long distances instantly and uses blood as a sacrifice.

So basically, the blood serves as a means of identifying the summoner for the summoned and also acts as a price (sacrifice) for calling them.