Under iron limiting conditions, A. pleuropneumoniae, like other members of the families Neisseriaceae and Pasteurellaceae, expresses two transferrin binding proteins, TpbB (approx. 60 kDa) and TbpA (approx. 110 kDa), enabling the organism to utilize porcine transferrin as the sole source of iron. Transferrin binding proteins have long been known to be important for bacterial virulence, but their role in infection is still not fully understood. For Neisseria spp. it is known that, in vitro, only the TbpA protein is required for utilization of transferrin whereas the TbpB protein shows different affinities to iron-saturated and apo-transferrin. The A. pleuropneumoniae TbpB protein also shows selective binding for iron-saturated transferrin and, in addition, confers a protective immune response.