1. An anti-CD123 single chain fragment variable (scFv) encoded by a nucleic acid comprising nucleotide sequences that encode the complementarity determining regions (CDRs) of a variable heavy (VH) domain and a variable light (VL) domain, wherein the VH and VL domains bind the alpha subunit of the interleukin-3 receptor (IL3Rα), and wherein the CDRs comprise the VH domain CDR1 of SEQ ID NO: 3, the VH domain CDR2 of SEQ ID NO: 4, the VH domain CDR3 of SEQ ID NO: 5, the VL domain CDR1 of SEQ ID NO: 8, the VL domain CDR2 of SEQ ID NO: 9, and the VL domain CDR3 of SEQ ID NO: 10.