Figure 1.

Immune and non‐immune barriers in the female reproductive tract (FRT). The FRT can be broadly divided into an upper section, comprising the uterus and the endocervix, and a lower section, which includes the vagina and ectocervix. The upper reproductive tract is lined by a single layer of columnar epithelium, while the lower reproductive tract is lined by stratified squamous epithelium. The zone where the two types of epithelia meet is called the transformation zone. Several non‐immune barriers form a first line of defence against pathogen invasion: the presence of tight junctions constitutes a physical barrier, mucus and antimicrobial peptides form a chemical barrier, and the Lactobacillus‐rich vaginal milieu creates a biological barrier. This multi‐layered defence strategy is further reinforced by a variety of immune cells that reside within the epithelium and the lamina propria (LP), patrolling for invading microorganisms. Pie charts indicate the composition of immune cells along the FRT. In the upper FRT, the composition differs by stage of the menstrual cycle or pregnancy where ‘Implantation’ represents the composition in the secretory phase of the menstrual cycle and early pregnancy. AMPs, antimicrobial peptides; DC, dendritic cell; FRT, female reproductive tract; g, granulocyte; ILC, innate lymphoid cell; LP, lamina propria; m, monocyte/macrophage; NK, natural killer.