We all despise seasonal flu—days spent in bed with a pounding headache, lethargy, fever, stuffy nose, coughing up colored mucus (Which you never expect to come out of you!), and the body's predicament between chills and sweating. The only silver lining during the flu is being pampered and enjoying some comforting chicken soup. That is if you can taste it!
I remember being down with flu most of the time when I was eight and nine. In my bedroom loft, my parents had placed a box of non-stick cookware with a picture of a delicious 'Bindi ki sabzi,' also known as a ladyfingers dish, along with other items. This image taunted me of the bland meals offered during my days in bed. Alone in my bedroom, as I lay with heavy eyes, a parched and sore throat, weak and exhausted, I wondered if this was what life was all about! Fortunately, years after, I could maintain perfect attendance at school. I was no longer susceptible to the clutches or, shall I say, the 'Spikes' of the seasonal flu. It was only a few years later that I learned about the immune system.
Our immune system is complex, and complex mechanisms protect us from being vulnerable to disease-causing agents. These agents include viruses, bacteria, fungi, protozoans, and parasitic worms. However, let me restrict my discussion to the immune response against viruses. From the above context, you must know by now that the discussion will include the influenza virus. But which influenza virus? You should know that only specific influenza virus strains can infect humans. Particular strains infect only birds or mammals such as swine or horses. In this blog, I shall not provide an in-depth account of the structure of the virus as it is totally out of scope. However, I will be concise on it to help you appreciate your immune system.
Influenza A is generally responsible for human pandemics. So, I shall anoint it, the 'villain' of the story! What does the villain look like? As we have seen in movies, villains usually don the most bizarre costumes, long-dark hoods, capes, bold jewellery, spikes, etc. Similarly, the villain of this story, Influenza A, is enveloped with 'Spikes' all over. The Spikes are the weapons comprised of Hemagglutinin and Neuraminidase. Within this envelope is the nucleocapsid, a protein layer that protects the genetic matter. All villains have weaknesses that can destroy them. The genetic material, eight single-stranded RNA, is the greatest weakness of this villain if targeted!
The villain is also a remarkable 'Con Artist.' As a part of their game, these con artists change their identities. But how does Influenza A do it? The villain alters the Hemagglutinin and Neuraminidase spikes protruding from its envelope. These changes can occur gradually over a period or suddenly.
Now, who protects us from these con artists, a.k.a. villains? Our complex immune system defends our body against the invasion of foreign particulate matter, including Influenza A. Hence, they have also been termed the 'Defence System.' The first line of agents alerting the defence system is the macrophages and the neutrophils. A warning is raised the minute the villain is detected. The alarm alerts the other agents of the defence system, including the T cells and B cells*. These agents are active in the villain's presence and produce memory cells. How does this entire defence operation unfold?
Remember, the eight-year-old me sick with flu in bed? I was a naive subject who hadn't encountered the villain before. The villain entered my body as I breathed in the aerosols where they were dormant. The villain's weapon, Hemagglutinin, clasped the receptors of my cells and then invaded them. Once the villain had proliferated within, they swarmed out using Neuraminidase to slash open my infected cells. The defence system was triggered to act as the villain adapted and multiplied within me. The Hemagglutinin spikes were easily accessible to the agents of the defence system, and the defence operation was initiated.
Upon the incursion by the villain, the T cells alert and activate the B cells. The B cell defence system has two wings- the short-lived plasmablasts and the memory cells. The plasmablasts generate the specialised weapons, ' Antibodies.' During the defence, antibodies increase in the body, preventing the villains from entering the cells. Simultaneously, the T killer cell wing kills the cells infested by the villain. Thus, the agents, T and the B cells work together to neutralise the villain. The activated immune response to the villain is what we experience as 'flu symptoms.'
Now that the villain's identity has been revealed, the memory cell wing documents it in their 'Villains' database.' Trained to recognise, the memory cell wing would swiftly activate when encountered by the familiar villain. The powerful fighters producing a surge of antibodies would swiftly thwart the invaders. Hence, saving you from that villain, influenza, once again!
I would fail if I did not clarify why I termed influenza A the 'Con artists.' In a constant game of deception, the villains mutate their appearances, altering their antigens** and leaving the memory cell wing of the immune system bewildered. These changes can leave the defenders unable to recognise the new identity of the villains, rendering their weapons less effective. The defence system flounders to keep up with these con artists, and individuals can become susceptible to sickness again. Also, a sudden, drastic transformation could lead to cross-species infections that unleash pandemics. I will delve into this matter another day.
References
Krammer, F. (2019). The human antibody response to influenza A virus infection and vaccination. Nature Reviews Immunology, 19(6), 383-397.
Punt, J., Stranford, S., Jones, P., & Owen, J. (2019). Kuby Immunology (6th ed.). W.H. Freeman and Company
Slifka, M. K., & Amanna, I. J. (2019). Role of multivalency and antigenic threshold in generating protective antibody responses. Frontiers in immunology, 10, 956.
*T and B cells originate in the bone marrow. However, the abbreviations T and B are Thymus and Bone marrow, where these cells mature.
**The molecule that activates the immune response is called the 'antigen.' Hence, Hemagglutinin is an antigen!
The weapons, antibodies are specific to the antigens. Any changes in the antigens leave the antibodies useless.
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