Neisseria acquires iron by extracting it from serum transferrin (TF) in its human host. Acts as a TF receptor and is required for TF utilization. Binds both apo- and holo-TF, via the TF C-terminus. Binds both human apo- and holo-transferrin (TF), via the TF C-terminus. Forms a large complex with TF and TbpB. The N-terminal periplasmic domain encodes a plug that inserts into the 22 beta-strand barrel, the extracellular loops extend up to 60 Angstroms away from the outer membrane. Part of the plug (the plug loop, resides 121-139) interacts with transferrin (TF), as does the L3 helix finger in extracellular loop 3 (residues 351-361). When the L3 helix finger inserts into TF it disturbs the conformation of TF and its coordination of iron 2. Electron microscopy suggests that in the TbpA-TbpB-TF complex, TF is captured directly above the loop domain of TbpA in a chamber of about 1000 Angstroms(3) formed by the 3 proteins, where interactions between the proteins serve to abstract iron 2 from TF. Present in outer membrane vesicle formulations which are used as vaccines in human. N.meningitidis cells will only bind to human TF, not bovine or porcine TF, explaining at least in part the bacteria's inability to cause infection in non-human hosts. Belongs to the TonB-dependent receptor family.