Cutting edge: terra firma: a retreat from "danger".

The danger model differs from the associative recognition model solely with respect to the origin of effector T helpers (eTh). More properly, the danger model should be juxtaposed to the Ag-independent model for the origin of eTh cells (i.e., the primer question). The introduction of danger in no way challenges the need to make a self-nonself discrimination, nor, for that matter, does it challenge the need to define self as those Ags encountered in the absence of eTh and that persist, whereas nonself are those Ags encountered in the presence of eTh and that are transient. The clarification of these points is our goal here.