A host organism needs time, often days, to mount an immune response against a new antigen, but memory cells permit a rapid respo
nse 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 trigger a rapid immune making it difficult or impossible to develop effective vaccines against them. Some pathogens, including HIV, have developed mechanisms to evade the immune system, 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. What strategy could a pathogenic virus use to evade the immune system? A. The virus can target, infect, and destroy immune system cells. B. The viral surface proteins mutate rapidly. C. The viral particles do not encode proteins. D. The virus does not produce antibodies. E. Viral particles prevent immune cells from making antibodies.
Viruses can evade the processing and presentation of antigens, for example by interfering with the expression of MHC class I proteins, although this increases their susceptibility to be detected by natural killer cells (NK). However, some members of the herpesvirus, papillomavirus, retrovirus, poxvirus and flavivirus families have also developed strategies to escape the attack of NK cells and promote their survival, inhibiting cell apoptosis. Finally, some pathogens often change surface antigens frequently, as in the case of influenza viruses (orthomyxovirus).
The human immunodeficiency virus (HIV) affects CD4 + T cells and degrades the host's ability to counterattack with a strong cell-mediated immune response. There are so many tactics of immune evasion used by HIV, which have so far hindered the development of an effective vaccine.
