Marsupials are born without a functioning adaptive immune system, into a non-sterile environment where they continue to develop. This review examines the extent of exposure of pouch young to microorganisms and describes the protective mechanisms that are complementary to adaptive immunity in the developing young. Complementary protective mechanisms include the role of the innate immune system and maternal protection strategies, such as immune compounds in milk, prenatal transfer of immunoglobulins, antimicrobial compounds secreted in the pouch, and chemical or mechanical cleaning of the pouch and pouch young.
Edwards, M., Hinds, L., Deane, E., & Deakin, J. (2012). A review of complementary mechanisms which protect the developing marsupial pouch young. Developmental Comparative Immunology, 37, 213-220. https://doi.org/10.1016/j.dci.2012.03.013