The pathways underlying dendritic cell (DC) activation in allergic asthma are incompletely understood. Here we demonstrate that adoptive transfer of ovalbumin-pulsed wild-type (wt) but not of C5a receptor-deficient (C5aR⁻/⁻) bone marrow (BM)-derived DCs (BMDCs) induced mixed T helper type 2 (Th2)/Th17 maladaptive immunity, associated with severe airway hyperresponsiveness, mucus production, and mixed eosinophilic/neutrophilic inflammation. Mechanistically, antigen uptake, processing, and CD11b expression were reduced in C5aR⁻/⁻ BMDCs. Further, interleukin (IL)-1β, -6, and -23 production were impaired resulting in reduced Th17 cell differentiation, associated with accelerated activated T-cell death in vitro and in vivo. Surprisingly, we found an increased frequency of CD11b(hi)CD11c(int)Gr1⁺F4/80⁺ cells, expressing arginase and nitric oxide synthase in C5aR⁻/⁻ BM preparations. Intratracheal administration of ovalbumin-pulsed wt DCs and sorted CD11b(hi)CD11c(int)Gr1⁺F4/80⁺ C5aR⁻/⁻ cells reduced Th2 immune responses in vivo. Together, we uncover novel roles for C5aR in Th17 differentiation, T-cell survival, and differentiation of a DC-suppressor population controlling Th2 immunity in experimental allergic asthma.