  We map a class of well-mixed stochastic models of biochemical feedback in
steady state to the mean-field Ising model near the critical point. The mapping
provides an effective temperature, magnetic field, order parameter, and heat
capacity that can be extracted from biological data without fitting or
knowledge of the underlying molecular details. We demonstrate this procedure on
fluorescence data from mouse T cells, which reveals distinctions between how
the cells respond to different drugs. We also show that the heat capacity
allows inference of absolute molecule number from fluorescence intensity. We
explain this result in terms of the underlying fluctuations and demonstrate the
generality of our work.
