HOST IMMUNITY TO VIRAL INFECTION

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Most viral infections do not result to disease development with the exception of HIV/AIDS and some haemorrhagic viral infections such as Ebola and Lassa fever virus infection that usually leads to serious disease development in the affected host and even death in some cases. In order words, an intact and strong immune system is critical to the effective containment and prevention of the further in vivo spread of ‘some’ viral infection. And after a proper containment of the viral infection by the host immunity; a long-lasting immunity (only for some type of viral infections) is usually developed in the infected individual or host in order to mount an effective and quick immune response to the second appearance of the same or similar infectious viral agent in the future.

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While some viral infections like Ebola and Lassa fever virus infection can result in acute disease development in the affected host, infections caused by HIV-1 usually do not result in an acute infection at the onset of infection; instead HIV-1 infection culminates into a slow, chronic and progressive infection that gradually depletes the infected hosts immune system, thus leading to AIDS after a protracted HIV-1 infection (usually about 10 years later). The effectiveness of the immune system of the viral-infected host or individual is one singular factor there is that can significantly affect the outcome of a particular viral infection in an individual. Even though the immune system may not eliminate totally the infecting pathogenic virus, a strong immune system helps to contain the pathogenicity or virulence of a given pathogenic virus. The administration of potent vaccines and other antiviral agents to affected hosts and other susceptible host can help to treat the disease or infection and even prevent the contamination of a particular viral infection respectively.

Vaccine development (which is mainly based on the isolation or cultivation, attenuation or inactivation and the direct or indirect injection of a susceptible host with killed, live or purified subunits of causative pathogenic microorganism) has saved mankind from the onslaught of some life-threatening infectious diseases such as measles and smallpox to mention a few, through the control and the complete eradication of these diseases in some cases.The timely vaccination of a large population of susceptible host against a particular viral infection can help to minimize the spread of the pathogen in the immunized populace through herd immunity. And vaccination/immunization has helped to increase the life expectancy of mankind through the protection of the population from life-threatening diseases especially those caused by pathogenic viruses. Viral infection unlike other microbial infections stimulate an immune response in the host; and this serves to protect the host from immediate attack and even from futuristic viral infection through specific immune responses.

Pathogenic viruses as obligate intracellular microorganisms engaged in uniquely intimate host-parasite relationships with the living organisms (plant, animal or man) that they infect, and this is due in part to the fact that viruses only exist or replicate in living host cells. In the course of their pathogenicity and/or virulence in a particular host, pathogenic viruses express gene products that act to circumvent one or more of the several antiviral defense mechanisms (for example, production of interferons) developed by the host organisms towards the invading pathogenic virus. Nevertheless, the host resistance to viral infections involves both the humoural immunity and cell-mediated immunity. While some viral infections (for example, influenza and common colds) can be contained by the host’s immune system; some others such as the causative agent of AIDS (i.e., HIV-1) overpowers the host’s immune system and makes it incapable to fight against the invading viral agent and other pathogenic microorganisms.

Infections with some pathogenic viruses may lead to apoptosis. Apoptosis is defined as programmed cell death. This programmed cell death is a host defense mechanism that can be inhibited by some viruses. Antibodies produced by humoural immunity can neutralize pathogenic viruses by interfering with their attachment to host cells; and the production of antibodies also enhances the destruction of viral particles through phagocytosis. However, antibodies cannot completely eliminate the infecting virus once the virion has incorporated its genome into that of the host cell. The cell-mediated immunity is one of the major important arms of the immune system that interfere with viral replication in vivo. Activated lymphocytes including cytotoxic T lymphocytes (CTL), helper T cells (CD4+) and cytotoxic T cells (CD8+) can recognize and initiate the process involved in the destruction of viral infected cells especially when these organisms bud off from their host cells.

But this is not always the case for some pathogenic viruses such as retroviruses (for example, HIV-1) that form latent infection, in which case, the CTL arm of the host immune system will be incapacitated (if not deficient) to effectively purge the infecting latent virus from its hiding place (known as reservoir sites) in the host cell.The production of interferons (which are protein substances produced by cells during viral infection) helps to reduce the spread of pathogenic virus in vivo especially in some benign viral infections such as influenza and cold. Interferons stimulate the production of natural killer (NK) cells and T cells. Interferons also accelerate the immune response of a host to viral infection. And by acting on other effector cells of the immune system, interferons (which are antiviral cytokines) generally reduce the susceptibility of other uninfected cells of the host to the invading pathogenic virus, and they do so by localizing the pathogenic virus so that they do not easily spread in the host’s body.           

Further reading

Acheson N.H (2011). Fundamentals of Molecular Virology. Second edition. John Wiley and Sons Limited, West Sussex, United Kingdom.

Brian W.J Mahy (2001). A Dictionary of Virology. Third edition. Academic Press, California, USA.

Cann A.J (2011). Principles of Molecular Virology. Fifth edition. Academic Press, San Diego, United States.

Carter J and Saunders V (2013). Virology: Principles and Applications. Second edition. Wiley-Blackwell, New Jersey, United States.

Dimmock N (2015). Introduction to Modern Virology. Seventh edition. Wiley-Blackwell, New Jersey, United States.

Kudesia G and Wreghitt T (2009). Clinical and Diagnostic Virology. Cambridge University Press, New York, USA. 

Marty A.M, Jahrling P.B and Geisbert T.W (2006). Viral hemorrhagic fevers. Clin Lab Med, 26(2):345–386.

Strauss J.H and Straus E.G (2008). Viruses and Human Diseases. 2nd edition. Elsevier Academic Press Publications, Oxford, UK.

Zuckerman A.J, Banatvala J.E, Schoub B.D, Grifiths P.D and Mortimer P (2009). Principles and Practice of Clinical Virology. Sixth edition. John Wiley and Sons Ltd Publication, UK.

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