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mAbs specific for surface proteins on APCs can serve as Ag-delivery vehicles that enhance immunogenicity. The practical use of such constructs is limited by the challenge of expressing and modifying full-sized mAbs. We generated single-domain Ab fragments (VHHs) specific for class II MHC (MHCII), CD11b, and CD36. VHH sequences were modified by inclusion of a C-terminal sortase motif to allow site-specific conjugation with various Ag payloads. We tested T cell activation using VHHs that target distinct APC populations; anti-MHCII adducts elicited strong activation of CD4+ T cells, whereas anti-CD11b showed CD8+ T cell activation superior to targeting via MHCII and CD36. Differences in Ag presentation among constructs were unrelated to dendritic cell subtype or routing to acidic compartments. When coupled to antigenic payloads, anti-MHCII VHH primed Ab responses against GFP, ubiquitin, an OVA peptide, and the α-helix of influenza hemagglutinin's stem; the last afforded protection against influenza infection. The versatility of the VHH scaffold and sortase-mediated covalent attachment of Ags suggests their broader application to generate desirable immune responses. [ABSTRACT FROM AUTHOR]