For any kind of message transfer, there needs to be messengers. In the body, these take the form of ligands. Ligands are substances in the body made out of amino acids. Their job is to bind with cells. There are three types:

• Neurotransmitters: the smallest ones, they carry information from one neuron to the next within the brain and throughout the nervous system. Their range is local: they transfer messages between neighbouring neurons.
• Steroids: these have been transformed from cholesterol into a specific type of hormone. Hormones are produced by one tissue and carried long distances by the blood stream to another targeted tissue. Receptors for these ligands are found in the endocrine system as well as in the brain and immune systems.
• Peptides: the most "popular" information substances. They are for mass communication. They can go around the neighbourhood and throughout all the body's systems: immune, endocrine, digestive, excretory and brain, i.e. everywhere!

The messages that ligands carry range from "breath in, breath out" to "don't move, snake ahead." Ligands keep the brain informed with what's happening in the body and vice versa.