Signalling molecules are chemical messengers (like hormones, neurotransmitters, growth factors) that transmit information between cells, enabling communication, coordination, and physiological responses in multicellular organisms, acting by binding to specific receptors on target cells to trigger internal changes, ranging from short-range local signals (paracrine) to long-distance travel (endocrine). They vary widely in structure, from small gases to large proteins, and are crucial for development, immunity, and maintaining homeostasis, with errors often linked to diseases.
