Hormones are chemical messengers that are produced in one part of an organism and travel through the bloodstream to another part of the organism where they exert their effects. Hormones are only able to affect target cells/organs because they have specific receptors that bind to them. These receptors are located on the surface of target cells or inside the cells. When a hormone binds to its receptor, it triggers a cascade of events that leads to a change in the cell's activity.

There are a number of reasons why hormones are only able to affect target cells/organs. First, hormones are typically produced in very low concentrations, so they are not present in high enough concentrations to affect cells that do not have receptors for them. Second, hormones are often rapidly metabolized or excreted from the body, so they do not have a long enough half-life to affect cells that are not constantly exposed to them. Third, hormones are often transported through the bloodstream bound to proteins, which prevents them from interacting with cells that do not have receptors for them.

The specificity of hormone action is essential for maintaining homeostasis in the body. If hormones were able to affect all cells in the body, they would cause chaos. For example, if insulin were able to bind to receptors on all cells, it would cause blood sugar levels to drop to dangerously low levels. Similarly, if adrenaline were able to bind to receptors on all cells, it would cause the heart rate and blood pressure to increase to dangerous levels.

The specificity of hormone action is a result of the evolution of the endocrine system. Over time, the endocrine system has evolved to produce hormones that are only able to bind to receptors on specific cells/organs. This specificity has allowed the endocrine system to precisely regulate the body's metabolism, growth, and reproduction.