A chemical messenger is a substance that transmits signals between cells in the body. These messengers play a crucial role in various biological processes, including growth, metabolism, and immune responses. Common types of chemical messengers include hormones, neurotransmitters, and cytokines, each serving specific functions in communication within the body. Hormones, such as insulin and adrenaline, are produced by glands and released into the bloodstream, affecting distant organs. Neurotransmitters, like serotonin and dopamine, are released by nerve cells to communicate with other neurons. Cytokines are involved in immune responses, helping to regulate inflammation and cell signaling during infections.