The interaction between polymer brushes and mesoscopic particles is investigated both theoretically and experimentally. We present an analytical mean-field theory for a polymer brush (a layer of long polymer chains end-grafted to a substrate) with varying excluded volume interactions between monomer units. This system mimics the reversible adsorption of mesoscopic particles, such as surfactant micelles or proteins, on the grafted chains. The equilibrium structural properties of the brush (the brush thickness and overall degree of complexation) as well as the number of adsorbed particles per unit area, , are analysed as functions of the affinity between particle and chain, grafting density and excluded volume interactions. In our model is found to have a maximum as a function of . Experimentally the adsorption of BSA on a hydrophobic substrate with grafted PEO chains is measured with reflectometry. In the case of short grafted chains the adsorbed amount of BSA, , decreases continuously with increasing , which agrees with previous results and model calculations in the literature. In the case of long PEO chains, however, is found to have a maximum as a function of . Qualitatively the experimental dependence of on is found to agree with the results of our mean-field model. PEO chains show no affinity for BSA in the bulk, whereas in a grafted conformation an effective attraction is found. Some comments are made on the nature of this affinity, which is not yet fully understood.