Characterization of CD8-specific single domain antibody. (A) Representation of a camelid heavy-chain-only antibody and a conventional IgG. The VHH portion is indicated. (B) Site-specific labeling of VHHs using sortase. (C and D) characterization of X118-VHH and Alexa647-labeled X118-VHH, where SDS-PAGE (C) and LC-MS (D) analysis confirming the identity of the final products (lane 1, marker; lane 2, VHH-X118; lane 3, VHH-X118-Alexa647). (E) FACS analysis of splenocytes and lymph nodes gated on CD45⁺CD19⁻CD3⁺ cells confirming that X118-VHH stains CD8⁺ cells. Results are representative of three to four experiments with similar results.