The Hitchhicker's Guide to Immunity

Jun 10, 2020
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The Hitchhiker’s Guide to Immunity

Just for the record this has nothing to do with hitchhiking but I thought it would get your attention. In discussing COVID-19 exposure, treatments and vaccines, people refer to establishing immunity against SARS-CoV-2, without clarifying how they are using the term immunity. What this virus does to the body with various symptoms and how patients are treated and recover eventually leads to some type of immunity. Immunity may be temporary and last for days, weeks, months or years such as the flu. Or immunity may last a lifetime such as with measles. As an aside, measles has the ability to wipe away 20-50% immunity to previous diseases leaving a person vulnerable to previous diseases for which they once had immunity.
So how immune am I?
When someone has immunity to COVID-19 this means different things to different people. When the WHO said having COVID-19 doesn’t mean you could never get it again the press jumped on that. What people hear, what they read, who they talk to and what research is being pushed through the media have been influencing people’s perception of immunity.
There are degrees of immunity. If you get infected with the same virus again it could be just as bad as the first time, you could get infected but have a milder case, it could be so mild you don’t even know you have it but you are still contagious and are spreading it to others, or you may never get infected again. There are no guarantees as everyone’s body reacts differently and this virus attacks differently. Many people believe if they get infected again it won’t be too bad because our bodies would be able to fight it before it gets out of hand. Do you think if you have immunity you are safe to be around vulnerable people because you can’t infect them? What degree of immunity would allow you to feel safe around others? There is no guarantee you would not shed virus and be contagious again, and whether this is related to the degree of immunity you have.
What types of immunity are there?
Natural Active Immunity occurs when we are exposed to a pathogen, develop a disease and then develop immunity against it such as measles, mumps, or influenza. This is also known as Adaptive Immunity and is unique to vertebrates.
Natural Passive Immunity occurs during pregnancy when antibodies are passed from mother to fetus through the placenta or through breast milk to an infant. This is temporary and helps infants fight infections for the first few weeks or months of life until its own immune system is fully developed. Breast feeding, when possible, is really important to the health of an infant. Natural Passive Immunity is also used when someone receives medical injections of gamma globulins.
Artificial Immunity occurs when the body is given immunity by providing intentional exposure to a disease with small quantities or parts of the pathogen, such as with a live vaccine or a killed vaccine. This occurs with diseases such as measles and the flu.
You can develop immunity against the measles or influenza by receiving the vaccines or by being exposed to the diseases. We know immunity to measles is for a lifetime, whether you are exposed to the pathogen and develop the disease or you receive a vaccination, whereas immunity to the flu lasts for about a year whether you get the flu vaccine or get the flu. In either case of the flu you are open to getting it again the following year.
How Does Immunity Develop?
We are born with innate immunity, a non-specific combination of physical, chemical, and cellular defenses to prevent pathogens from entering our body. Think of tears when you get something in your eye, sneezing or coughing when you get something in your nose or lungs, and fevers. Other organisms in nature also have innate immunity, think of plants that are poisonous to eat or the tentacles of a jellyfish.
Our white blood cells, known as leukocytes, are either phagocytes that surround and absorb pathogens and effectively eat them or they are lymphocytes known as B and T cells. The adaptive immune system involves the production of B and T cells. These cells are both made in the bone marrow but must mature to become activated. The letters represent where in the body these cells mature, B cells mature in the bone marrow while T cells are transported and mature in the thymus gland. As B cells or T cells are produced, they are cloned to specifically target the pathogen they have detected. Once matured they live longer in our bodies providing that long-term immunity we all hope for, so we don’t get infected with the same disease twice. As B cells produce antibodies, these remain in our blood and provide protection from future infections.
B cells fight bacteria and viruses by making antibodies which lock onto their target and hang on until other immune cells can destroy the invader. There are two types of T cells, Helper T cells and Killer T cells. Just as the names imply, Helper T cells communicate with other cells particularly triggering the B cells to produce antibodies and alerting other leukocytes, while Killer T cells destroy cells that have already been infected. Yes, they kill our own cells to get rid of invaders and work particularly well fighting viruses.
Distinctions Without Value
If you have a similar disease to let’s say COVID-19 and get cross immunity from a different pathogen is that natural or is that artificial? Does it matter?
What Does Matter
How long immunity lasts and the degree to which it protects you does matter.
Understanding the type of immunity and what triggers each is critical to understanding why an alternative to vaccination with a competitive disease could be beneficial against SARS-CoV-2 and COVID-19.

Sources
How The Immune System Works:
https://www.medicalnewstoday.com/articles/320101

Classifying Immunities Lumen Boundless Microbiology
https://courses.lumenlearning.com/boundless-microbiology/chapter/classifying-immunities/

The History of Vaccines:
https://www.historyofvaccines.org/content/articles/passive-immunization
 

MMohammed

Assistant Community Manager
Staff member
Nov 12, 2019
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An articulate, nuanced take on immunity. The distinctions you make between the different types of immunity is really useful to remember.
 

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