- Hormones: These are secreted by endocrine cells and travel through the bloodstream to distant target cells. They play essential roles in growth, metabolism, and homeostasis.
- Neurotransmitters: Released by neurons, these chemicals cross synapses to transmit signals to other neurons, muscle cells, or glands.
- Cytokines: These small proteins are crucial for cell signaling in the immune system, helping regulate inflammation and immune responses.
