Introduction: The amniotic fluid – as the medium surrounding the fetus, it is holding a crucial role in the maintenance and development of a successful pregnancy. While providing mechanical protection to the fetus, it also offers considerable immunological defense. In fact, it is known that the amniotic fluid plays a significant role in the innate immune system, as many of its corresponding substances show substantial antimicrobial function. Also, components of the adaptive immune system, including B cells, have been described within the amniotic fluid. An increase of immune cells in the amniotic fluid in cases of intra-amniotic infection indicates their involvement in inflammation-related pathologies of pregnancy. However, especially B cells in the amniotic fluid have not yet been thoroughly investigated. The aim of this work is a deeper examination of the B-lymphocytes within the amniotic fluid. Based on the analysis of surface molecules this includes their phenotype, origin and func-tion. In the long term this could substantiate our understanding of intraamniotic inflammation and or infection, which are casually linked with preterm birth, fetal inflammatory response syndrome and fetal morbidity. This, in turn, could pave the way for potential diagnostic methods and treatments. Methods: For all experiments 8-12-weeks-old pregnant mice were sacrificed at day 14 of pregnancy. The amniotic fluid was collected and specific cell subsets were isolated using MACS cell separation ...