A tetrameric protein complex consisting of two platelet-derived growth factor (PDGF) receptor subunits and two PDGF ligand subunits. Binding of the PDGF ligand dimer to the PDGF receptor in the plasma membrane induces receptor dimerisation and activation. PDGFs are involved in a wide variety of signalling processes and are found in all vertebrates. At least two different receptor chains (A and B) and four types of ligand chains (A, B, C, and D) are known forming a wide variety of combinations of receptor-ligand complexes. [ http://www.ncbi.nlm.nih.gov/pubmed/11331882 ]