A current goal of cell biologists and immunologists consists of relating the behavior of cellular systems to the physical properties of their molecular components. An ab initio simulation of a living cell in the spirit of molecular dynamics is not feasible (Netz and Wheaton, Pnas 118:e2022753118, 2021), and we would be unable to analyze the flow of data such a simulation would generate. Therefore, there is a need for a coarse-grained account of the molecular properties of cell components, and also of cell behavioral patterns (e.g sticking to a surface, starting a differentiation program, sending forward a filopodium …). Clearly, there is a trade-off between the refinement and tractability of the parameters we define to account for cell structure and function.

The aim of this talk is to review a number of attempts at relating the properties of immune cell receptors to their function with a particular emphasis on the early steps of T cell activation. Indeed, cells continually probe their environment with membrane receptors in order to retrieve cues that enable them to make decisions. It is therefore warranted to ask which set of parameters may be sufficiently exhaustive to account for the molecular properties and function of a given cell, and also to define a cell decision.

First, we briefly review a few well known models of immunological interest to define the time scale and mechanical conditions of receptor-ligand interactions.

Second, we review the growing complexity of the parameters that were used during the last decades to describe receptor binding properties : affinity, binding kinetics, force dependence, time and force dependence of these properties.

It is concluded that a more exhaustive description of binding properties might be useful in the future, and currently available tools yield more and more precise connection of these properties with atomic properties of involved molecules.