A model for parvovirus B19 binding and entry into primary human erythroid cells. Mature human RBCs, which express high levels of P antigen receptor, allow virus binding but not viral entry because they lack the α5β1 integrin coreceptor (A), whereas erythroid progenitor cells, which express both P antigen receptor and α5β1 integrin coreceptor, are permissive for parvovirus B19 entry (B).