Physiological principle of ligand-receptor specificity, what establishes and what is it used for?

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This video describes the principle of ligand-receptor specificity, which establishes that each endogenous or exogenous ligand acts on its own or specific receptor that generates a specific cellular response. Ligands are chemical messengers that can be endogenous (produced by the organism itself) such as e.g. eg neurotransmitters, hormones and cytokines or they can be exogenous such as odors (odoriferous substances) and flavors (tasters). The receptors for all of these chemical messengers are highly specific. These receptors are found on target cells. This physiological principle is fundamental for the communication between the cells of the organism. Therefore, it is key for the nervous, endocrine and immune systems. It also plays a fundamental role in sensitivity, especially when it involves chemoreceptors such as the senses of taste and smell. Water-soluble ligands have membrane receptors, and lipid-soluble ligands have intracellular (cytosolic or nuclear) receptors. Membrane receptors can be of the receptor-channel type (eg Na, K or Cl channel), they can be associated with G protein and activate second messengers (cAMP, cGMP, IP3, DAG and Ca) or they can act as enzyme receptors. Enzymes adenylate cyclase, guanylate cyclase, phospholipase C. Intracellular receptors act as gene transcriptors. In addition, its most common applications are described in detail. This principle is very useful for teaching - learning cell communication, nervous, endocrine and immune activity, antigen recognition, olfactory and taste sensitivity, and even in the union of the egg with the sperm (fertilization).