Hormones are chemical messengers secreted into blood or extracellular fluid by one cell that affect the functioning of other cells.
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. In other words, a particular cell is a target cell for a hormone if it contains functional receptors for that hormone, and cells which do not have such a receptor cannot be influenced directly by that hormone. Reception of a radio broadcast provides a good analogy. However, in order to be a NPR target and thus influenced directly by their broadcasts, you have to have a receiver tuned to that frequency.