Most hormones circulate in blood, coming into contact with essentially all cells.
However, a given hormone usually affects only a limited number of cells, which are called target cells.
A target cell responds to a hormone because it bears receptors for the hormone.
These receptors are proteins that have a pocket where the hormone or part of it fits. This triggers a cascade of events that triggers the hormone's effects on the cell.
These receptors are either on the cellular plasma membrane or are inside the cell (usually the nucleus).
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