Before revision:  The theory allows us to calculate an effective order parameter, temperature, external field, and heat capacity from T cell fluorescence data without fitting. Experiments agree with the scaling predictions, suggesting that this type of nonequilibrium criticality plays an important role in cell biology .
After revision:  The theory allows us to calculate an effective order parameter, temperature, external field, and heat capacity from T cell flow cytometry data without needing to know the underlying molecular details. We find that T cells obey critical scaling relations and exhibit critical slowing down, and that the heat capacity determines molecule number from fluorescence data. We demonstrate that critical scaling holds even as our system is driven out of its steady state, via the Kibble-Zurek mechanism for driven critical systems. Our approach places a ubiquitous biological mechanism into a known class of physical systems and is immediately applicable to other biological data .