Down-regulation refers to the process by which a cell decreases the amount of specific receptors or proteins on its surface in response to external signals. This can occur in various biological processes, such as hormone regulation, immune response, and cell signaling. Down-regulation often involves the internalization and degradation of receptors or proteins, leading to a decrease in the cell's responsiveness to a particular stimulus. This process is crucial for maintaining homeostasis and proper cellular function in the body.