The mechanisms associated with the induction of systemic immune responses by nanoparticles are not fully understood, but their elucidation is critical to address safety issues associated with the broader medical application of nanotechnology. In this study, a key role of nanoparticle-induced exosomes (extracellularly secreted membrane vesicles) as signaling mediators in the induction of T helper cell type 1 (Th1) immune activation is demonstrated. In vivo exposure to magnetic iron oxide nanoparticles (MIONs) results in significant exosome generation in the alveolar region of Balb/c mice. These act as a source of nanoparticle-induced, membrane-bound antigen/signaling cargo, which transfer their components to antigen-presenting cells (APCs) in the reticuloendothelial system. Through exosome-initiated signals, immature dendritic cells (iDCs) undergo maturation and differentiation to the DC1 subtype, while macrophages go through classical activation and differentiation to the M1 subtype. Simultaneously, iDCs and macrophages release various Th1 cytokines (including interleukin-12 and tumor necrosis factor α) driving T-cell activation and differentiation. Activated APCs (especially DC1 and M1 subtypes) consequently prime T-cell differentiation towards a Th1 subtype, thereby resulting in an orchestrated Th1-type immune response. Th1-polarized immune activation is associated with delayed-type hypersensitivity, which might underlie the long-term inflammatory effects frequently associated with nanoparticle exposure. These studies suggest that nanoparticle-induced exosomes provoke the immune activation and inflammatory responses that can accompany nanoparticle exposure.