hormone receptor
A hormone receptor is a protein that works like a lock waiting for the right key. A hormone drifts past countless cells, but only the cells carrying the matching receptor can read its message. The hormone fits into the receptor, and that snug fit is what tells the cell, “this message is for you.”
More precisely, a hormone receptor binds its hormone selectively and with high affinity, then changes shape. That shape change is the trigger: it sets off a chain of events inside the cell that ends in a specific response, such as switching on a gene, opening an ion channel, or activating an enzyme. Receptors fall into two broad families based on where they sit — on the cell surface for water-soluble hormones, or inside the cell for fat-soluble ones.
Receptors explain why one hormone can do different things in different tissues: the response depends on the receptor and the machinery the cell connects to it, not just on the hormone. They also explain certain diseases — if a receptor is missing or faulty, the hormone can be present in normal or even high amounts yet the body acts as if it were absent, a state called hormone resistance.
Insulin circulates to nearly every cell, but muscle and fat respond strongly because they carry abundant insulin receptors; cells with few receptors barely react to the same blood level.
Same hormone, different responses — set by which cells carry the receptor.
Affinity (how tightly a receptor grips its hormone) and number (how many receptors a cell has) both shape sensitivity. A cell can tune its responsiveness by changing receptor number even if hormone levels stay constant.