  Chemical signaling is one of the ubiquitous mechanisms by which
inter-cellular communication takes place at the microscopic level, particularly
via chemotaxis. Such multi-cellular systems are popularly studied using
continuum, mean-field equations. In this letter we study a stochastic model of
chemotactic signaling. The Langevin formalism of the model makes it amenable to
calculation via non-perturbative analysis, which enables a quantification of
the effect of fluctuations on both the weak and strongly-coupled biological
dynamics. In particular we show that the (i) self-localization due to
auto-chemotaxis is impossible. (ii) when aggregation occurs, the aggregate
performs a random walk with a renormalized diffusion coefficient $D_R \propto
\epsilon^{-2} N^{-3}$. (iii) the stochastic model exhibits sharp transitions in
cell motile behavior for negative chemotaxis, behavior which has no parallel in
the mean-field Keller-Segel equations.
