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How Our Immune System Fights COVID-19

Until a vaccine is developed, people are depending on their immune systems — “the multi-level defence network” — as the only biological defence available to protect them against COVID-19.

The Immune System

The immune system is unique in its own way because it is the only system that has no official organs. The main parts of the immune system are leukocytes (white blood cells or WBC) and antibodies, which work with the lymphatic system. The lymphatic system is composed of the spleen, the thymus, and the bone marrow. All of these elements work together in order to protect the human body from infection.

Diagram of the immune system (Health Direct)

The Immune Response

The immune response is a defence mechanism in which the body recognizes and defends itself against pathogens such as bacteria, viruses, and any other foreign substance. The immune response consists of two processes: the innate response and the adaptive response. The innate response is one all humans are born with, while the adaptive response is developed with age.

Let’s explore their other differences…

Diagram showing how the body’s defence against pathogens works. Firstly, a non-specific response occurs, where non-specific cells attack a foreign substance whether the body recognizes it or not; however, this is a weaker attack. If it is a foreign substance that is not recognized, the specific response adapts to it and B cells create antibodies that could attack it. This is stronger than the non-specific response but it is slower. (Intact Schools)

Other Important Cells and Their Roles

CD4+ (T helper cells): White blood cells that signal to other immune system cells how and when to fight foreign pathogens and infections.

CD8+: Cells that release two types of cytotoxic (toxic to living cells) protein, “granzymes”, which are able to induce apoptosis (death of a cell) in any type of target cell.

Cytokine: A type of signalling molecules that regulate immunity, inflammation and hematopoiesis (production of blood cells and plasma)

Chemokine: A subcategory of cytokines that control the positioning and movement of immune cells, control the release of non-specific immune cells from the bone marrow in response to infection and inflammation but also during homeostasis.

How does this apply to COVID-19?

The first immune response is very quick. Humans have innate immune cells that sense the virus as a pathogen, and they start producing cytokines to stop the virus. A couple of these cytokines kill cells in order to stop the virus from replicating. An immune response is the first step of defence against any pathogen - a virus in the case of COVID-19. However, the excessive inflammatory innate response as well as a damaged adaptive host immune defence can cause tissue damage all throughout the body of the host, instead of specifically where the virus is located.

Several studies have displayed important changes happening both in non-specific and specific immune systems in patients with COVID-19. In particular, lymphocytopenia - the condition of having an abnormally low level of lymphocytes in the blood - and modulation in total neutrophils seem to be directly correlated with the severity of the disease and death. In patients with severe COVID-19, scientists have observed a decrease in the levels of CD4+ and CD8+ cells, B cells, NK cells, monocytes, eosinophils and basophils.

When looking at the roles of these cells, it is logical to conclude that a decrease in the levels of CD4+ and CD8+ cells means that it will be difficult to signal to other immune system cells how and when to fight SARS-CoV-2 or release the cytotoxic proteins that kill target cells. A decrease in B cells signifies that it could be difficult to produce antibodies that are specialized to eliminate the virus. And finally, the decrease of non-specific immune response cells might slow down or stop the non-specific response, which is the first reaction the body does to protect itself, meaning the body will not be able to perform the first reaction that will be able to save it.

***It is important to note that the previous paragraph has conclusions made by the writer of this article and are not backed up by any sources***

Additionally, most of the patients severely affected by COVID-19 showed a great increase in serum levels of pro-inflammatory cytokines (cytokines that worsen the disease). An increase in serum cytokine and chemokine levels in infected patients has been correlated with the severity of the disease, suggesting a possible impact for hyper-inflammatory reactions in COVID-19 pathogenesis (development of the disease).

Why are older people more likely to die from COVID-19?

As people grow older, both specific and non-specific response cells become less efficient at protecting the body and responding to an infection.

In the innate immune cells, the production of cytokines stops working properly as it starts taking a longer time to produce them. By the time they’re ready, the pathogen may have already attacked the body’s cells a significant amount. Also, the innate immune cells don’t communicate as effectively with the adaptive immune cells in order to start the more specialized line of defence.

It is also important to know that immune cells are derived from stem cells in the bone marrow. As people grow older, the bone marrow has a smaller amount of these cells to use, which impacts how many immune cells the body produces.

Article Contributors: Celine Guirguis, Rahma Osman, Valerie Shirobokov


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