ACE2 and infection: In the so-called “Up-state” of the RBD, the (prefusion) protein is able to bind to ACE2 (Angiotensin Converting Enzyme 2) and infect (via a transformation to its fusion state) human epithelial cells (Type I and II pneumocytes; also, alveolar macrophage and nasal mucosal cells), but in the “Down-state” of the RBD the Spike protein is believed to be inactive to ACE2 binding and to cellular infection.