Embryo implantation is only possible within a very short window of time in the female cycle. At that time, a perfect synchronisation of the uterus and the embryo is critical. Immunological processes can be detected very early in the female cycle.

After ovulation, during the luteal phase (second phase of the cycle) uterine NK cells (natural killer cells) begin to settle in the endometrium. These differ from NK cells in the blood, as they have no cytotoxic (cell-killing) activity. The trophoblasts (cells of the placenta) begin to grow deep into the maternal tissue and also into the vascular walls during implantation and placenta maturation. Maternal blood vessels are opened to allow direct blood contact with the trophoblasts.

This process is controlled by uterine NK cells and other cells, as well as by immune messengers in the endometrium. A perfect immune balance is essential for the success of a pregnancy.

Any disruption of the TH1/TH2 immune balance (balance between cellular and humoral immune response) will result in miscarriages or a disruption of the placenta maturation, leading to complications during pregnancy, or may prevent a successful implantation of the embryo in the first place.
Specific immunological conditions, like Hashimoto’s disease (thyroid disease), rheumatism, coeliac disease, and others will create an imbalance of the immune system towards TH1. Some cellular immune disruptions, like increased numbers of NK cells or an elevated T4/T8 index are of equal significance. The same goes for the functioning of NK cells (controlled by the presence of activating or inhibiting receptors; KIR genotype).
A healthy pregnancy is characterised by continuous apoptosis (controlled cell death) and the clearance of apoptotic cells (removal out of dead cells) by macrophages (scavenger cells) at the fetal maternal border between mother and fetus. Healthy immunological processes are of critical importance here.