The aim of this clinical trial is to investigate the change in endometrial composition during the window of implantation following intrauterine administration of PBMC immunomodulated with IFNt. Patients seeking assisted reproductive therapy will be invited to participate. Two endometrial biopsies will be obtained from each patient during mid-secretory phase of two consecutive menstrual cycles. The first biopsy will be obtained one month before the intervention, and the second one - a day after intrauterine administration of the tested cell treatment which will take place the following month. Immunohistochemistry analysis of the cell composition of the endometrium will be performed.