A host organism needs time, often days, to mount an immune response against a new antigen, but memory cells permit a rapid response to pathogens previously encountered. A vaccine to protect against a particular viral infection often consists of weakened or killed virus or isolated proteins from a viral protein coat. When injected into a person, the vaccine generally does not cause an infection and illness, but it effectively teaches the immune system what the viral particles look like, stimulating the production of memory cells. On subsequent infection, memory cells recognize and bind to the virus and a rapid immune response. Some pathogens. including HIV. have developed mechanisms, to the immune system, making it difficult or impossible to effective vaccines against them. Assume that a host's antibodies and T-cell receptors are available to bind to any structure that might appear on the surface of a pathogen and that, once bound, the pathogen is destroyed,

Required:
What strategy could a pathogenic virus use to evade the immune system?