PF4 and bacterial infectious disease: However, none of them had reported the activation of either Tfrc, a gene encoding a cell surface receptor necessary for transferrin recycling and iron uptake, or Pf4. Transferrin is one of the acute-phase proteins that are produced by the liver in response to extracellular bacterial infections, which in turn produce iron-sequestering and a nutritional immunity (Parrow et al., 2013).