In this episode, Normal Cell gets bored being in charge of cell proliferation and is annoyed by Killer T Cell’s shouting. Just as he lies on his bed, he investigates a strange noise, finding Virus-Infected Cell hiding from Killer T Cell. After laughing from Killer T Cell getting punched by a glove-in-the-box, Normal Cell accompanies Virus-Infected Cell to pull pranks on other cells. Taking Virus-Infected Cell to his room, he realises that Virus-Infected Cell wants to infect him and gasps in horror as the local area is filled with viruses. Suddenly, Killer T Cell and the other immune cells arrive to beat up the virus-infected cells. Killer T Cell then berates Normal Cell for troubling the other cells and advises him to study more on viruses and cells.
This episode describes a well-known viral infection called the common cold. What is the common cold and how does it infect people in real life? Join us as we delve more into this common viral infection.
What is the common cold?
The common cold is a viral infection of the upper airway, infecting the nose and throat. A widespread popular belief of the common cold is that it can be caught by being wet or chilly such as running through the rain without an umbrella. However, various studies have shown that this is not the case. In fact, outdoor temperatures do not affect the likelihood or severity of the common cold. Hence, they can be caught anywhere and any time of the year, though they appear more frequently in autumn and winter. On average, adults get 2-4 colds/year and children 6-8 colds/year.
The common cold can be caused by more than 200 viruses. The most common viruses that cause the common cold are rhinoviruses, coronaviruses, parainfluenza viruses, respiratory syncytial virus (RSV), adenoviruses and metapneumoviruses. The most common virus that causes the common cold, and one that will be further discussed, is the rhinovirus.
Rhinovirus
| Schematic | Microscopic | Anime |
 |  |  |
Rhinoviruses, first discovered in the 1950s, are classified under the Picornaviridae family and Enterovirus genus and are split into three groups: A, B and C. Rhinoviruses are icosahedral-shaped and are among the smallest viruses at a diameter of 30nm. Inside the virus is a single RNA strand that is 7.2 kilobases long. This RNA strand contains a single gene coding for a long protein that is split into 11 separate proteins inside a virus-infected cell. Of these, VP1, VP2, VP3 and VP4 make up the viral capsid that covers the RNA genome.
Rhinoviruses enter the body through the nose or the eye (where it drains to the nose via the lacrimal duct). Rhinoviruses initially travel to the back of the nose to infect respiratory epithelial cells. As the virus binds to the cell, the viral RNA is injected into the cell. From there, viral RNA and proteins are rapidly produced and assembled inside the infected cell. New rhinoviruses emerge from infected cells 8-10 hours after entry into the body. Unlike most viruses, rhinoviruses do not kill epithelial cells, though they detach them from the respiratory epithelium. This disrupts the epithelial barrier which allows rhinoviruses and other pathogens from the nose to infect other cells.
The immune response to the common cold
The immune system is activated when receptors on white blood cells engage with rhinovirus capsid proteins and RNA. This leads to the production of interferons that limit viral replication and pro-inflammatory cytokines that attract and activate immune cells such as neutrophils and promote inflammation. This is shown in the episode when Killer T Cell and other immune cells are mobilised upon detection of viral RNA in the glove-in-the-box (seen in the picture above).
The adaptive immune response to rhinoviruses is mostly mediated by antibodies, particularly IgG in the blood and IgA in the nose. These antibodies act to neutralise and mark rhinoviruses for phagocytosis as well as kill virus-infected cells. In terms of the cellular immune response, helper T cells secrete cytokines and promote antiviral responses against the common cold. Even though killer T cells were mobilised in the episode to kill virus-infected cells, they have not been explicitly studied, so their role in common cold infections has not yet been verified.
How do symptoms of the common cold arise?
Symptoms of the common cold arise in response to viral infection of the upper airway and immune responses to the virus. These symptoms peak at 1-3 days after infection and most people recover within 7-10 days.
Usually, the first sign of a common cold is a sore throat as the virus initially infects the upper respiratory tract. Chemicals such as prostaglandins and bradykinin are released which irritate sensory nerves in the throat, generating the sensation of pain. The nose is also blocked as large veins around the nose dilate to block air-filled cavities (sinuses) around the nose. Mucus and plasma also drain into the nose, producing a runny nose.
The person may also sneeze as inflammation activates nerves leading to the sneeze centre of the brain. The sneeze centre engages facial and respiratory muscles to breathe in and sneeze out the virus. Coughing is usually the last symptom to appear in a common cold. Coughing may arise due to inflammation from the nose irritating nerves around the larynx (voice box) in the throat, initiating the cough reflex.
Other symptoms of the common cold may include fever (common in children but rare in adults), headaches and body aches due to the release of pro-inflammatory cytokines that act on the brain to produce the symptoms.
Treating the common cold
Common cold infections force people to stay home from school or work which produces an economic burden. It is estimated that in the US in 2003, common colds costed the health care system $17 billion and the economy $22.5 billion in lost productivity. This drives the development of vaccines and treatments for the common cold to prevent people from being infected and affected by the infection.
Unfortunately, because the common cold is caused by many different viruses (and even different strains of rhinovirus), there is currently no cure or vaccine for the common cold. The best way to treat a common cold is to get lots of rest, drink plenty of fluids and to not go to school or work. Medicines can help ease the symptoms of the common cold, but they do not speed up the elimination of the virus. These include aspirin and ibuprofen (Nurofen®) which relieve pain and fever and decongestants combined with antihistamines which ease a blocked nose.
There have been many myths and old wives’ tales of how to prevent and treat the common cold such as vitamin C and steam inhalation. However, most of them are ineffective in preventing or relieving the symptoms of the common cold.
Conclusion
The common cold is a viral infection of the upper airway that appears frequently, particularly in autumn and winter. They are mostly caused by rhinoviruses which are among the smallest and simplest viruses. They infect respiratory epithelial cells and activate immune responses which together produce symptoms such as sore throat, runny nose and sneezing. Even though these viruses do not kill cells, they can still replicate very rapidly inside infected cells, appearing a short time after entering the body. It is quite lucky; therefore, that Normal Cell is spared of infection. Hopefully he will learn his lesson next time!
This is the last post of the first season of “The Science behind Cells at Work!” series. I hope you have enjoyed reading these posts as much as I have researching and writing them. See you then!