The mammary gland is intolerant to bacterial intrusion

Mammals depend on the secretion of milk to rear their offspring, which exposes the organ in charge of the function, the mammary gland (MG), to bacterial threat. The essential driving force that conditions the interactions of bacteria with the MG is the abundant secretion of milk, a nutritious fluid which endows the common mastitis-causing pathogens with a doubling time of less than 30 min. From this angle, mammals rely on a potential bacterial bioreactor for the survival of their offspring. The MG is lined with a two-layered epithelium devoid of protective mucus. This means that the mammary epithelium is exposed directly to bacteria once they have passed through the opening lactiferous canal. To cope with the threat, the MG resorts to neutrophilic inflammation to check bacterial proliferation in its lumen and at its epithelial lining. Promptness of neutrophil recruitment is a necessity, which requires a low threshold of activation on the part of the mammary epithelium. Constrained by natural selection, the MG has evolved an innate and adaptive immunity intolerant to bacteria regardless of their level of virulence. The evolutionary issue has been to find a compromise between the deleterious tissue-damaging side effects of inflammation and the maintenance of the secretory function indispensable for the offspring’s survival. It appears that the MG relies mainly on neutrophilic inflammation for its protection and is regulated by type 3 immunity. Advances in knowledge of type 3 immunity in the MG will be necessary to induce immune protection adapted to the physiology of this peculiar organ.