Answer:
Hormones.
Explanation:
Hormones have a really low concentration in blood but are able to trigger a large response because they bind their receptors with extreme affinity, this means that even when there are just a few molecules, they will bind the receptor. Once this interaction happens, the receptor passes the information to the inside of the cell, interacting with many other molecules that continue passing the message to generate a response. Thanks to this, a single hormone can trigger interactions between hundreds of other molecules that amplify the signal, causing a large biological response.
