Effects on the hadronic and electromagnetic properties of the two-nucleon system above pion threshold, arising from the interaction of the Δ isobar with nucleons, are investigated. The instantaneous nucleon-Δ potential is based on the meson exchange. Two-body reactions connecting channels with at most one pion and one photon are studied. Processes leading to a three-body pion-two-nucleon final state are considered in the restricted kinematic domain in which the pion forms the P33 resonance with one of the nucleons. The nucleon-Δ potential is seen to increase the relative importance of the inelastic strength of two-nucleon spin-triplet states with respect to spin-singlet states, correcting a deficiency common in most existing models. Theoretical predictions are compared with recent experimental data for the various reactions. In particular, the differential cross section and the proton beam asymmetry for pp→nΔ++ (pπ+) are calculated, the latter observable being especially sensitive to the nucleon-Δ interaction.