Lymph Nodes: The Unrecognized Barrier against Pathogens.

Lymph nodes have been studied for decades as the main site of the adaptive immune response. In this Viewpoint, we outline how the lymph nodes have another less appreciated function as an active innate barrier. Lymph nodes drain lymphatic fluid from tissues that are exposed to the external environment, such as the skin, lung, or gut. Pathogens that travel through lymphatics should be able to enter the circulation, if it were not for the strategic localization of lymph nodes along lymphatics which prevent systemic access. There is growing evidence for several populations of innate immune cells in the lymph node that function to control pathogens. Understanding how the lymph node functions as an active innate barrier can contribute to improving defenses against dissemination of infections in patients.