  We present a so-called adaptive Ising model (AIM) to provide a unifying
explanation for sensitivity and perfect adaptation in bacterial chemotactic
signalling, based on coupling among receptor dimers. In an AIM, an external
field, representing ligand binding, is randomly applied to a fraction of spins,
representing the states of the receptor dimers, and there is a delayed negative
feedback from the spin value on the local field. This model is solved in an
adiabatic approach. If the feedback is slow and weak enough, as indeed in
chemotactic signalling, the system evolves through quasi-equilibrium states and
the ``magnetization'', representing the signal, always attenuates towards zero
and is always sensitive to a subsequent stimulus.
