Proposed mechanisms by which maternal exposure to bacteria protects against allergies in offspring. Aerosol exposure to microbe-containing dust particles induces mild-to-moderate inflammation in the lungs, including increased expression of TLRs and production of cytokines. Cytokines might then enter the bloodstream and be delivered directly to the placental tissues, where they depress TLR expression, cytokine production, and influence resident myeloid cell functions. Circulating cytokines might also enter the maternal bone marrow, where they stimulate and “program” myeloid precursor cells. Programmed DCs and monocytes could then enter the circulation, and some of these cells could traffic to the decidua, where they could replace resident myeloid populations and influence the local inflammatory milieu.