How does the immune response get started?

An effective adaptive immune response requires the prior induction of the regulatory effector T-helper (eTh). There are two competing models of how this cell is induced to effectors. Under the Associative Recognition of Antigen (ARA) or “two signal” model, the T-helper requires eTh in order to be induced to eTh, an “autocatalytic” process. Under the “costimulation” model eTh are induced by an antigen-unspecific signal derived from an “activated” APC. Under the ARA model the problem of the origin of the primer eTh is posed. A nonself antigen-independent pathway to eTh is proposed as well as an experiment to reveal its existence. In the costimulation framework no primer eTh need be postulated but it lacks a mechanism that, in the absence of ARA, accounts for the self–nonself discrimination and the determination of effector class.