target cell

A hormone broadcast into the blood reaches nearly every cell, yet most cells ignore it completely — the way a public-address announcement washes over a whole crowd but only the named person reacts. A target cell is one that carries a receptor able to recognize that hormone, so it alone is wired to respond.

A target cell is any cell that expresses receptors specific for a given hormone and is therefore capable of responding to it. Whether the receptor sits on the cell surface or inside the cell depends on the hormone's chemistry. The same hormone may have many target tissues — cortisol acts on liver, muscle, immune, and brain cells — because each of them displays its receptor.

The strength of a cell's response depends not just on hormone concentration but on how many receptors it has and how active they are; cells can turn responsiveness up or down by changing receptor numbers. A caveat: when receptors are lost or malfunctioning, a target cell may become deaf to a hormone even when blood levels are high — the basis of hormone resistance, as seen in type 2 diabetes.