Overview: There are four basic categories of chemical signaling found in multicellular organisms: paracrine signaling, autocrine signaling, endocrine signaling, and signaling by direct contact. The main difference between the different categories of signaling is the distance that the signal travels through the organism to reach the target cell. I will mention an example of “Endocrine Signaling” which affects the Whole Organism: For example, cells in the adrenal gland produce the hormone adrenaline, which causes the body to enter the "fight or flight" mode. The hormone spreads throughout the body in the blood and causes reactions in targeted cells. Blood vessels constrict to increase blood pressure for the muscles, the heart pumps more quickly and some sweat glands are activated. The whole organism is placed into a state of readiness for extra exertion. The pathway of signaling transduction: In endocrine signaling, the originating cell secretes a hormone that is stable over long distances. The hormone diffuses through the cell tissue into capillaries and travels through the circulatory system of the organism. Endocrine hormones spread throughout the body and target cells in locations that are remote from the signaling cell. The targeted cells have receptors for the hormone and change their behavior when the receptors are activated. Ligands and receptors: Ligand is the binding of a signaling molecule. Receptor is the receiving molecule. Receptors and ligands come in many forms, but they all have one thing in common: they come in closely matched pairs, with a receptor recognizing just one (or a few) specific ligands, and a ligand binding to just one (or a few) target receptors. Binding of a ligand to a receptor changes its shape or activity, allowing it to transmit a signal or directly produce a change inside of the cell. cellular response: The many different ways cells can change their behavior in response to a signal. Cell signaling pathways vary a lot. Signals (a.k.a. ligands) and receptors come in many varieties, and binding can trigger a wide range of signal relay cascades inside the cell, from short and simple to long and complex. Despite these differences, signaling pathways share a common goal: to produce some kind of cellular response. That is, a signal is released by the sending cell in order to make the receiving cell change in a particular way. In some cases, we can describe a cellular response at both the molecular level and the macroscopic (large-scale, or visible) level. Defects in Signaling Pathways Can Lead to Cancer and Other Diseases: In light of their complexity, it comes as no surprise that signal-transduction pathways occasionally fail, leading to pathological or disease states. Cancer, a set of diseases characterized by uncontrolled or inappropriate cell growth, is strongly associated with defects in signal-transduction proteins. Indeed, the study of cancer, particularly cancer caused by certain viruses, has contributed greatly to our understanding of signal-transduction proteins and pathways.