Viral immunity

In the past year, our lives have all been disrupted because of the pandemic caused by the Covid-19 virus. But what is it that actually happens within our bodies and immune systems when we encounter such a virus?

There is truly a sense of beauty and intelligence associated with the human immune system’s ability to recognise, fight and establish memory immunity against viruses which invade the body.

The location and type of host cell favoured by a virus depends on the type of virus. For example, human influenza – the virus typically responsible for seasonal epidemics – gains access to upper respiratory tract cells whereas HIV invades a variety of immune cells. Viruses are entirely dependent on host cell machinery to be able to replicate and so after invading a host cell, viruses unload their nucleic acids and the host cell is coaxed into using this genetic material to produce more viral particles. The new viruses then leave the host cell – sometimes killing the host cell in the process – and are ready to invade new host cells.

Host cells have receptors both on their surface and inside the cell which can recognise viral nucleic acids and protein. Viral protein also ends up being displayed on the host cell surface which signals to the immune system that the host cell is virus-infected. In response to immune cells recognising viral material, they secrete signalling molecules called ‘interferons’ which interfere with viral replication. The particular helper T lymphocyte cells which have the correct receptors to bind to the viral antigens can recognise viral protein displayed on host cells and activate specifically those B lymphocyte cells which also have the correct receptors tor the viral antigens. The activated B cells mature and produce antibodies which bind to viruses and prevent them from entering more host cells. Cytotoxic T lymphocyte cells with the specific receptors for the viral antigen respond to viral proteins displayed on host cells by killing virus-infected cells. This prevents more viruses from being released.

The clever thing is, once the immune system has seen a particular virus, it can form an immunological memory because some helper T cells, cytotoxic T cells and B cells which responded to the virus the first time become ‘memory cells’ which survive beyond the initial infection and patrol the body. When the same virus invades the body again in the future, a swift and efficient immune response can eliminate the virus before symptoms of illness even emerge.

So why do people get ill with some viruses repeatedly many times in their life? Viruses have some tricks up their sleeve too, one of which is a high rate of mutation in viral nucleic acids which leads to a change in viral antigens such that they can no longer be recognised by memory cells. In the case of influenza, something even worse can happen – entire segments of influenza nucleic acid from different types of influenza can recombine to form radically new influenza strains the likes of which have never been seen by the human immune system.

For example, human, avian and swine strains of influenza recombined to form the H1N1 virus which caused the 2009 swine flu pandemic. Unfortunately, there are things which can go wrong with the immune system’s response to viruses; e.g. the response may be ‘over-zealous’. If a virus has antigens similar in structure to a human protein, the immune system may learn to fight against the human body itself, causing autoimmune disease.

There is much still to learn about viral immunity. The Covid-19 pandemic is yet another example in history which illustrates the dangers viruses can pose to us. With advances in our understanding, hopefully we will be better prepared for such crises in the future.

Further reading:

Unraveling the Mystery of Immunity

Andreas Halner
Andreas Halner

Andreas Halner, 4th year Medicine student at St John’s College

I started studying Medicine at St John’s College, Oxford, in October 2015. Since I was a kid, I’ve always loved maths so I decided to start a DPhil (PhD) in medicine and machine learning after completing preclinical medicine in 2018. Once I finish my DPhil, I’ll complete Medicine at the Oxford Clinical School. My DPhil research revolves around using data science to research different temporal patterns of physiology, biology and patient symptoms in lung disease. Combining my passion for medicine and maths with my social impact-focused entrepreneurial ambitions, I co-founded the start-up Oxford Cancer Analytics in 2019 which uses machine learning and blood tests to detect cancer early. Since school days I’ve been keen to teach, and am now a tutor for 2nd year medical students at St John’s in Pathology. I also do outreach work with secondary school and sixth-form students – for example, helping with Oxbridge interview preparation. Outside of work, my main interests centre around religion, philosophy and sport. My main sport focus is cross country running – although it’s the running I like, not the cold or the mud! 😃