Due to large equilibrium fluctuations of protons at physiological pH, the orthophosphate ion as well as the imidazole group on histidine substantially regulate their charge upon approaching charged interfaces. This implies that these-and comparable ions function as electostatic "proximity switches" when interacting with lipid membranes, DNA, proteins, etc. Using straightforward statistical thermodynamics as well as mesoscopic computer simulations we quantify the charge regulation mechanism and argue that it is important in a range of biological as well as technical processes.