  Cells often have tens of thousands of receptors, even though only a few
activated receptors can trigger full cellular responses. Reasons for the
overabundance of receptors remain unclear. We suggest that, in certain
conditions, the large number of receptors results in a competition among
receptors to be the first to activate the cell. The competition decreases the
variability of the time to cellular activation, and hence results in a more
synchronous activation of cells. We argue that, in simple models, this
variability reduction does not necessarily interfere with the receptor
specificity to ligands achieved by the kinetic proofreading mechanism. Thus
cells can be activated accurately in time and specifically to certain signals.
We predict the minimum number of receptors needed to reduce the coefficient of
variation for the time to activation following binding of a specific ligand.
Further, we predict the maximum number of receptors so that the kinetic
proofreading mechanism still can improve the specificity of the activation.
These predictions fall in line with experimentally reported receptor numbers
for multiple systems.
