Abstract
The aim of this study was to find out how a long‐term shielding of the geomagnetic field (GMF) affected the immune system of rats. Male and female Wistar rats were kept up to an age of 2 months in a natural GMF (about 37 µT). Afterwards, the rats were divided into four groups (males and females separately): control rats were maintained in ambient GMF, while experimental animals were housed under conditions of a weakened GMF (below 12 µT) achieved with steel cages. After 6 months, the rats were sacrificed by decapitation. Spleens and thymuses were isolated and weighed. Peritoneal cells were eluted and cultured in vitro to study their ability to produce nitric oxide (NO) and to synthesize superoxide anion (O^−^~2~), important microbicidal molecules of macrophages. The number of macrophages was estimated by a crystal violet staining method. We found that the long‐term shielding of the GMF could influence the functioning of the immune system in a sex‐dependent manner. The deprivation of the GMF delayed physiological thymus involution, that effect being more strongly expressed in females. The weakening of the GMF resulted in an increased number of peritoneal macrophages, especially in males. The shielding of the GMF diminished the ability of macrophages to release NO and to synthesize O^−^~2~, those effects being more powerfully expressed in males and females, respectively. It is proposed that the observed changes in the immune system occur as a consequence of the protective effect of GMF shielding on the circadian rhythm‐dependent level of melatonin. Bioelectromagnetics 30:21–28, 2009. © 2008 Wiley‐Liss, Inc.