Panel 21. The entry of a virus into a cell always occurs through a specifi c receptor, similar to a key that corresponds to a very particular lock. The capsid proteins recognize the receptor on the target cell surface if the virus is naked, while the envelope proteins are responsible of such association in the case of an enveloped virus. This complementarity between a given viral protein and a receptor on the cell surface determines the virus’s ability to infect a particular cell in the body. Thanks to this complementarity, the virus attaches to the surface of a cell, as here in the case of a herpes simplex virus (HSV).