On how the immune system preferentially interacts with antigen-specific molecules bound to antigen over unbound molecules, with emphasis on B cell receptor signalling

Antigen-specific molecules of the immune system, namely antibodies, the membrane immunoglobulins (mIgs) of B cells and T cell receptors (TcRs), can all signal their interaction with antigen. There are different mechanisms by which this signalling could occur. These mechanisms can be divided into two general categories: allosteric and non-allosteric. In allosteric mechanisms, the monovalent binding of the antigen to the receptor triggers a conformational change at the binding site that is propagated to an invariant part of the receptor, a change recognized by a sensing unit. We argue allosteric mechanisms are implausible. Non-allosteric mechanisms depend on steric effects due to the antigen's size and/or multivalency. We consider two non-allosteric mechanisms by which the mIg of B cells has been envisaged to signal its interaction with antigen: the popular cross-linking model and the dissociation activation model. We argue, on the basis of both experimental observations and physiological considerations, that the dissociation activation model, developed by Reth and his colleagues, is uniquely plausible.