Immune system evading

One of the paradoxes about HIV infection is that most infected individuals contain HIV antibodies, but the disease eventually occurs in most cases, even in the presence of these antibodies. This means that HIV antibodies are unable to prevent the onset of AIDS. This may be due to several factors. First, the levels of antibodies raised might be insufficient to block the spread of infectious virus. In addition, antibodies can be produced against different parts of virus. Only some of these antibodies (neutralizing antibodies) can inactivate virus and prevent infection. Finally, several unique features of HIV infection provide the virus with ways to evade the immune system. 

Ideally, the most effective prevention of HIV infection would be a vaccine that blocks virus infection in individuals. Indeed, effective vaccines have been developed against most human viruses that cause serious diseases. While several different possible vaccines against HIV are under development, there are some theoretical reasons why it may be difficult to develop an effective one. First, HIV has the unique ability to evade the immune system in an infected individual. Briefly, this results from (1) the high mutation rate of the virus, particularly in the env gene (2) the ability of the virus to establish a latent state in some cells and (3) the ability of the virus to spread by cell-to-cell contact. The object of the vaccine is to raise a protective immune response to the infectious agent. Since HIV evades the immune system so efficiently, it may be difficult for a vaccine to prevent HIV infection in an individual, even if it can induce production of neutralizing antibodies or cell-mediated immunity. 

Box 17.6 Strategies employed by pathogens to evade detection or killing by the immune system... 

Influenza A infections are responsible for 300-500,000 deaths and 3-5 million hospitalizations per year. Every epidemic brings about new strains of influenza A, which arise due to point mutations within the surface glycoproteins hemagglutinin (HA) and neuraminidase (NA). These mutations in turn enable emerging virus strains to evade the host s immune system. The long production time of the current commercially available vaccine, which is produced in chicken eggs, is a major obstacle. 

African sleeping sickness, or African trypanosomiasis, is caused by protists (single-celled eukaryotes) called trypanosomes (Fig. 1). This disease (and related trypanosome-caused diseases) is medically and economically significant in many developing nations. Until recently, the disease was virtually incurable. Vaccines are ineffective, because the parasite has a novel mechanism to evade the host immune system. 

An unusual feature of the herpes viruses is their preference for nerve cells. Stress to the nervous system, such as emotional stress or sunburn, can cause the virus to replicate, which leads to an outbreak through the skin. When they are not replicating, these viruses remain in nerve cells where they are not detected by the body s immune system, which has little activity in nerve cells. While remaining dormant in the nervous system, the viruses evade not only the immune system but antiviral drugs as well. 

Clonal Expansion of B Cells Plasma Cells 1866 Box 31 -E Evading the immune System... 

Viruses use a large variety of mechanisms to evade cellular defense mechanisms. Almost every aspect of the innate or adaptive immune systems provides some opportunity for evasion.)) The rapid... 

Flatworm parasites are well recognized for their ability to live for decades in environments where they are in contact with potentially damaging immune factors. This adaptability reflects the fact that these parasites have evolved mechanisms to evade immune effector mechanisms, and more remarkably, to sense and utilize components of the host immune system for their own development. Schistosomes, digenetic trematodes, are undoubtedly the most well-studied parasitic flatworms. These parasites infect over 200 million people in tropical and subtropical zones, and cause severe disease in approximately 5% of those infected. Of immunological interest is the fact that pathology in schistosome-infected individuals is caused largely by the immune response... 

Viruses, especially large DNA viruses like poxviruses and herpesviruses, have evolved several mechanisms to evade, subvert, and manipulate the host immune system, ensuring their survival and spread (Murphy, 2001 Nash... 

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