ABSTRACT
It is essential that the immune system be tightly regulated. It has to be “turned on” in response to a “danger” signal, then fine-tuned to give an optimum and appropriate response (in the case of the adaptive immune response), and then “turned off” again when the danger has gone. The danger signals come mostly from invading microbes but also from within the body in the form of self antigens when the natural mechanisms for prevention (immunological tolerance) break down (Section L). In addition, some non-microbial substances such as allergens can pose as danger signals to some individuals and danger/alarm signals are generated within the body from “aberrant” cell death or cell stress resulting from a range of causes such as anoxia. All of the immune cells alerted in response to these danger signals have to be controlled and this involves a variety of regulatory mechanisms.
