Mechanisms of chemoattraction underlie the spatial organization of the cells of the immune system under basal conditions and the localization of these cells to sites of inflammation. The chemokines, a family of around 50 small proteins, play a major role in these processes. Leukocytes are equipped with cell-surface sensors for chemokines. There are 19 such receptors that are differentially expressed on leukocytes: the repertoire of receptor expression depending on the type of leukocyte and its stage in maturation. From observations in animal models, clinical studies, in vitro cell biology, and molecular analysis, a working hypothesis has been established to explain the cellular interactions underlying allergic responses and the chemokines–chemokine receptors involved. Chemokines signal through G protein–coupled receptors that are used typically for sensory functions (eg, detection of olfactory signals in the nose). This type of receptor can be blocked selectively by small-molecule antagonists. This provides the opportunity for the development of therapeutic compounds designed to suppress the recruitment of particular leukocyte types in allergic reactions.