Explain that receptors are specific proteins whose function is to receive chemical signals from outside the cell. Once received, the messenger signal sets up a cell-tissue-organ response which is a change in the electrical conductivity of the cell. The response can be an increase in capillary permeability, itching, vasodilation, an inflammatory response, increase in heart rate, increase in blood pressure, and when severe, anaphylaxis. Discuss the minimum receptor binding required for a response to occur.