In adolescents with endometriosis receiving progestogens for 1 year, a higher proportion of cytotoxic lymphocytes with the CD56dimCD16bright phenotype (p = 0.049), a higher level of CD206+ monocytes (p < 0.001) and a lower level of anti-inflammatory CD163+ monocytes (p = 0.017) was observed, which likely mediated the clinical effect of reducing the number of endometrioid heterotopias during therapy. This evidence concerns the gene MRC1 and endometriosis.