Influenza viruses are a virus that infects the respiratory tract of mammals and usually can cause fever, muscle pain, severe headache, sore throat, fatigue, coughing, and weakness. It is the Orthomyxoviridae family of enveloped viruses and a significant cause of respiratory infections worldwide (Bahadoran et al., 2016). It can be transmitted through the air by coughing or sneezing and also from infected birds through their droppings. Other transmissions are through saliva, nasal secretions, faeces, and blood of infected animals. Influenza viruses have four types which are A, B, C, and D. Influenza viruses have two proteins on it which help the infections to happen in mammals which are hemagglutinin (HA) and neuraminidase (NA). Hemagglutinin works as a mediator for the entry of the virus into the cell by membrane fusion activity and receptor binding while neuraminidase mediates the release of the virus by enzyme cleavage of the viral receptor (Bahadoran et al., 2016) .
Human influenza viruses that can cause seasonal epidemics of the disease are influenza A and B viruses. Other than that, influenza C viruses are generally can cause mild illness while influenza D viruses primarily affect cattle and not infect people. However, only influenza A viruses can cause flu pandemics (Centers for Disease Control and Prevention [CDC], 2019). This is because influenza A viruses can undergo evolutionary changes from one year to the next to evade immune protection of the host while maintaining their ability to function and interact with host cellular factors. Typically, there are two ways or mechanisms which influenza A viruses can change include antigenic drift and antigenic shift.
Antigenic drift or mutation is small changes in the virus structure or genome which can lead to changes in the surface proteins of the virus which are hemagglutinin and neuraminidase and will create new virus over a long time. This way take a long time to change enough for unrecognizable by antibodies of the immune system because minor changes of antigenic drift typically create viruses that are closely related to each other. As a result, these closely related viruses typically have identical antigenic properties that can be recognized by immune system antibodies (CDC, 2019). That is why, when a high case of virus infection happens in one year, the other years do typically not have a high case of infection than before. However, as minor variations occur over time or after a few years and result in antigenically different, immune system antibodies are no longer able to identify these new viruses, which can result in a person becoming vulnerable to influenza infection again as the virus has changed enough (CDC, 2019). Antigenic drift is the biggest explanation of why people may get the flu more than once, and the explanation of why the flu vaccine needs to be revised and changed regularly to keep up with evolving influenza viruses.
Next, antigenic shift or re-assortment is a major change of influenza A virus that can combine genome become new hemagglutinin and/or new hemagglutinin and neuraminidase proteins in influenza viruses and managed to infect the same cell (CDC, 2019). It drastically changes in virus structure or genome over a short time. This way usually may lead to pandemic infections. One way of shifting can happen is when the influenza virus from the animal population is able to infect humans which can lead to a pandemic of influenza A viruses. Pandemic influenza A viruses may also be Mproduced by reconfiguration between avian influenza viruses and human influenza viruses for the generation of new or novel viruses (Shao, W., Li, X., Goraya, M., Wang, S., & Chen, J.-L., 2017). This changes cannot be recognized by antibodies or memory cell because it has been totally modified as it has new strains and/or sub-types. This pandemic infections can cause the bad symptom of flu to humans as they do not have immunity against this new virus.
Some people might not get the flu even when the virus has successfully breached the physical barriers of that person. This is because they have a strong defence in their body that can protect them. Human bodies have three types of defence in their body which are physical defence, chemical defence, and cellular defence. Even though the virus can penetrate the physical defence, they may not successfully evade chemical and cellular defence for some people. Chemical defence of innate nonspecific immunity uses chemical mediators that have been detected in body fluid and tissue to help prevent microbial invaders. For example, interferons which are a class of cytokines are important in defence against viruses. It will inhibit viral replication and the development of mature viruses and hinder the spread of the virus. On the other hand, cellular defence such as leukocytes is primarily involved in the immune responses (Lumenlearning, n.d.). Subdivided leukocytes, such as lymphocytes, are very effective in releasing antibodies that bind to the virus and prevent it from infecting cells. Antibodies will bind to the surface protein of the virus called hemagglutinin (Lee et al., 2019).
Factors that viruses have failed to escape the person’s immune system are due to their diet, memory cells from the first infection they had, age and chronic illness. People who have a very high level of antioxidant precursors in their blood by drinking the juice, and eating fresh fruits and vegetables may protect them from getting sick with the flu (Taylor, 2011). Moreover, if the person has memory cells of the same strain that infect people on that year, they also may not get the flu. Furthermore, older adults have a higher risk of getting sick from the flu than young adults because their immune system was not as strong as young adults’ anymore. Ageing can gradually contribute to alterations in the immune system. As a result, specific symptoms that indicate the response of the immune system to these infections typically do not occur in older adults. Older adults, for instance, will not have a fever when they are sick with influenza. Some of their organs often experience some damage due to chronic illness or ageing. Thus, all these mechanisms for maintaining stability and work in the face of illness become less successful (Kernisan, 2018).
In conclusion, influenza viruses have four types which are A, B, C, and D but the only influenza A viruses can cause flu pandemics because it can change its structure or genome by antigenic drift or antigenic shift. The antigenic shift typically causes flu pandemics because influenza A viruses can change it structure in a very short time and produce a new virus that cannot be recognized by the antibodies or memory cells of the immune system. Whereas antigenic drift is a mutation of viruses that have little changes in their structures and take a long time to fully alter their structure to escape antibodies or memory cells, but antigenic drift can also be a factor why people can get flu more than once over time. Human bodies actually have protection against microbial invaders or pathogen. There are three types of defence in their body which are physical, chemical and cellular defence. That is why some people might not get the flu even though the virus has successfully breach physical defence. There are factors that have effectively prevented influenza viruses from penetrating the immune system, such as diet, memory cells from the last infection they had, and age. People who are young adults and have high antioxidant precursors usually have less risk to get the flu. Thus, it is very important to take flu vaccines to help strengthen the immune system against influenza viruses. Vaccines can function by activating an immune reaction to the virus so that human bodies are still secure when they encounter it.
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