Menstrual blood contains immune cells with inflammatory and anti-inflammatory properties

Aim Successful pregnancy requires balanced regulation of immune cells at the feto‐maternal interface. Systemic monitoring of the immune system cannot precisely outline local immune status in the uterus. In this survey, endometrial immune milieu was investigated using a non‐invasive method of analysis of menstrual blood (MB). The results were compared with peripheral blood (PB). Method PB and MB of healthy fertile women (n = 15) were collected simultaneously on the second day of their menstrual cycle. T and natural killer T cell subpopulations were immunophenotyped by flow cytometry. Results Among examined cell populations, the frequency of CD4 + Foxp3+, CD4 + Foxp3 + CD25‐, CD4 + Foxp3 + CD25+ and IL17+ T cells (P = 0.022, 0.028, 0.017 and 0.005, respectively) and TCRαβ+, CD45RO+, CD16‐, IFNγ + and IL17+ NKT (CD56 + CD3+) cells (P = 0.010, 0.037, 0.038, 0.015 and 0.021, respectively) were significantly higher in MB compared with PB. Conversely, PB contained a higher percentage of CD16+ T cells (P = 0.025) in comparison with MB. Conclusion MB contains cells of an inflammatory and anti‐inflammatory nature, implying the existence of finely tuned cell homeostasis during menstruation. Our results imply that MB could be viewed as an easy‐to access specimen for monitoring endometrial immune cells, especially those that have preferential endometrial localization.