Nanoparticles travel through blood vessels to reach disease sites, but the local environment they encounter may affect their surface chemistry and cellular interactions. Here, we found that as nanoparticles transit through injured blood vessels they may interact with a highly localized concentration of platelet factor 4 proteins released from activated platelets. The platelet factor 4 binds to the nanoparticle surface and interacts with heparan sulfate proteoglycans on endothelial cells, and induces uptake. Understanding nanoparticle interactions with blood proteins and endothelial cells during circulation is critical to optimizing their design for diseased tissue targeting and delivery.