Not all virus infections produce a protective immune response. These persistent viruses evade immune control by sequestration (hiding away); blocking antigen presentation; cytokine resistance; evading natural killer cell activity; escape from apoptosis (cell death), and antigenic shift (changing surface proteins). HIV evades the immune system by constantly changing the amino acid sequence of the proteins on the surface of the virion. Other viruses, called neurotropic viruses, move along nerves to places the immune system cannot reach.