Nanobodies as secondary antibodies. (a) Structure of the variable domain of an alpaca heavy chain antibody (V_HH). The variable loops are emphasized in yellow (PDB accession number 3OGO). (b) Structure of an IgG antibody with main nanobodies binding to F_ab or F_c domains to rabbit or mouse antibodies described in the manuscript. Inset shows comparative size of nanobody (PDB accession number 1IGT).