  Diffusion-limited reactions (DLR) are usually described within the
Smoluchowski theory, which neglects interactions between the diffusing
components. We propose a first extension of such frame- work that incorporates
excluded-volume effects, considering hard diffusing agents in the presence of
an absorbing sink. For large values of the absorber-to-diffuser size ratio Rs,
the encounter rate obtained from the simulations is in very good agreement with
a simple generalization of the Smolu- chowski equation at high densities.
Reducing Rs, the rate gets substantially depressed at interme- diate packing
and become even non-monotonic for Rs << 1. Concurrently with the saturation of
the rate, stationary density waves set in close to the absorbing center. A
mean-field, effective-potential analysis of the Smoluchowski equation under
crowding sheds light on the subtle link between such ordering and the slowing
down of the encounter dynamics. Finally, we show how an infinitesimal amount of
non-reacting impurities can equally slow down dramatically the reaction.
