Abstract
Successful immune defense is a complex balancing act. In order to protect a host against invasion by harmful pathogens, an immune response must be rapid and vigorous, and must eliminate foreign invaders before their populations grow beyond control. That same immune response, however, must be selective enough to recognize and ignore commensal bacteria, environmental antigens and host tissue itself. How the immune system makes the crucial decision whether or not to attack a particular antigen has been a long-standing question central to the study of immunology. Here we show that the structure of the signaling network between regulatory T-cells and type 17 helper T-cells allows the immune system to selectively attack pathogens based on whether or not the pathogens represent a growing, and thus dangerous population. We term this mechanism for immune system activation the 'Growth Detection Paradigm', because it offers an entirely new explanation for immune system regulation and peripheral tolerance.