The mucosal immune system of the reproductive tract is unique since it must not only be able to recognize and tolerate sperm during impregnation and the developing fetus, both of which have foreign protein markers. It must also be able to respond to a myriad of bacterial exposures and viral diseases. Studies are now showing the impact of the immune system on all components of the reproductive cycle, from follicular development to maintaining pregnancy. While many of these studies have been done in species other than cattle, they provide valuable insight into the influence the immune system has on reproduction in mammals.