Our immune system develops while under a constant barrage of microbes. Understanding how microbial encounters shape immunity is essential to treating immune dysfunction and infection. By contrasting mice raised under 'clean' laboratory conditions with mice raised under 'dirty' conditions we found natural microbial exposure dramatically influences early life immune development, expanding immune cells by over 4-fold, and improving host defense. Further, we discovered that this immune expansion was partially fueled by microbial exposure induced increases in the number of  immune cell progenitors (hematopoietic stem cells and progenitors).  

The immunology of pregnancy is complex and delicately balanced. While failure to tolerate fetal antigens during pregnancy can result in fetal loss, ineffective immunity to pathogens can threaten the survival of both the fetus and the mother. Fetal tissues and the maternal immune system continuously communicate to maintain this balance. Extracellular vesicles have been identified as an important mode of communication between maternal and fetal tissues. These vesicles bleb off of cells carrying potent combinations of  immunomodulatory proteins and mRNA transcripts to communicate across distant tissues and through barriers. Fascinatingly, maternal immune cells have also been observed to cross the placental boarder and take up residence in fetal tissues, where they can remain for the life of the offspring. The role and impact of these cells on offspring immune development and function remains unclear. 

The immune system plays a central role in all stages of pregnancy from implantation to birth. Pregnancy is an amazingly complex process that requires a lot of things to go right to reach term. Immune origins have been described for many pregnancy complications including,  pre-term birth, miscarriage, preeclampsia, and gestational diabetes.  

For infants, infection is the primary cause of hospitalization. Infants are at greater risk of infection because their immune systems have not yet developed. Importantly, maternal immunity can be transferred to offspring during gestation and through lactation to provide crucial supplemental protection while the infant immune system matures. Better understanding of the process of immune development, the benefits and limitations of the infant immune system, and how maternal immunity can be transferred are key to reducing dangerous infections during early life.