Int. J. Environ. Res. Public Health 2010, 7 950 modulations of 0.5, 3, 25 and 35 Hz [74]. It was subsequently shown that both the intensity and the orientation of the earth's magnetic field during exposure can alter these effects at specific frequencies. These and similar [75] results led to the development of ion resonance models for biological effects of EMF [37,38,74,76]. Figure 7. Possible mechanisms of the extremely low frequency EMF effects on transcription through Ca2+-operated pathways. (From [77], with modifications; reproduced with the kind permission of Dr. Maercker). It was proposed that membrane-mediated Ca2+ signaling processes are involved in the mediation of the electromagnetic field effects on the immune system [78]. Application of low frequency electromagnetic fields produce parallel shifts in Ca2+ uptake and DNA replication intensity in stimulated lymphocytes [79]. Exposure of immune cells to static magnetic field resulted in decrease of phagocytic activity, inhibition of mitogenic response to Con A in lymphocytes and enhancement of apoptosis in thymic cells [80]. Together, the data demonstrate the possibility of Ca2+ signaling-mediated immunomodulating effects of exposure to magnetic fields. Thus, calcium signaling and Ca2+-dependent transcriptional network represents another possible target of electromagnetic field influence on biological functions and host-pathogen interactions.