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Question Video: Applying Knowledge of Primary and Secondary Immune Responses to Novel Coronavirus Infections Biology

The disease COVID-19 is caused by infection with a novel virus strain called severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) as there is no instance of it being identified as infecting humans before 2019. Using your knowledge of primary and secondary immune responses, which of the following statements is true? [A] When it started to infect humans, SARS-CoV-2 only elicited a secondary immune response that was rapid and reduced transmission. [B] SARS-CoV-2 is a zoonotic disease, which means it cannot elicit a secondary immune response. [C] When it started to infect humans, no populations had any immunity and all individuals infected with SARS-CoV-2 died. [D] When it started to infect humans, SARS-CoV-2 only elicited a primary immune response that was slow and allowed the virus to spread quickly.

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Video Transcript

The disease COVID-19 is caused by infection with a novel virus strain called severe acute respiratory syndrome coronavirus two, SARS-CoV-2, as there is no instance of it being identified as infecting humans before 2019. Using your knowledge of primary and secondary immune responses, which of the following statements is true? (A) When it started to infect humans, SARS-CoV-2 only elicited a secondary immune response that was rapid and reduced transmission. (B) SARS-CoV-2 is a zoonotic disease, which means it cannot elicit a secondary immune response. (C) When it started to infect humans, no populations had any immunity and all individuals infected with SARS-CoV-2 died. Or (D) when it started to infect humans, SARS-CoV-2 only elicited a primary immune response that was slow and allowed the virus to spread quickly.

This question asks us about primary and secondary immune responses to SARS-CoV-2. Let’s remove the options for now so we can review the key facts about the adaptive immune system.

The adaptive, also called acquired or specific, immune system generates an immune response that is specific to the pathogen. This type of immunity uses both B and T cells or lymphocytes to fight and memorize a specific pathogen. There’re three main types of T cell: T-helper cells, which help the B cells and other T cells respond; cytotoxic T cells, which kill infected body cells; and suppressor T cells, which shut the immune system down after the infection has been cleared. B cells produce antibodies against the pathogen. Adaptive immunity involves the primary and secondary responses, so we will look at each of these in turn.

A primary immune response occurs when an antigen is encountered for the first time. An antigen could be part of a pathogen, foreign molecule, or toxin that the immune system recognizes as nonself. In response to the invasion, the T cells will help stimulate the B cells to produce antibodies against it. An antibody is a Y-shaped protein molecule with antigen-binding sites. As this is the first time the B and T cells have encountered this particular antigen, they have to be activated, a process known as clonal selection, so they can multiply, known as clonal expansion. The population takes time to come large enough to get the infection under control, so the response is quite slow. During the invasion, memory B and T cells are also produced, which will recognize this particular antigen if they come across it again.

The secondary immune response is a reaction to types of antigens that have previously infected the body. This response is much faster. And the antibodies are produced in much greater quantities than in the primary immune response. This is because the memory cells are already primed to recognize the antigens.

COVID-19 is a zoonotic disease as the SARS-CoV-2 virus is believed to have jumped from bats to humans via another animal. It emerged in 2019, which means that it did not infect humans prior to then. Because humans would not have encountered SARS-CoV-2 prior to 2019, their bodies would not contain memory cells against the virus. Upon encountering the virus for the first time, the human immune system would initiate a primary immune response, which is much slower than a secondary immune response. However, if an individual were to recover from COVID-19 and be reexposed to SARS-CoV-2, their body would initiate a faster secondary immune response.

Now that we have reviewed the key facts about the primary and secondary immune responses, let us return to the question. Since we know the virus causing COVID-19 would only elicit a primary immune response initially, we are able to answer our question. The correct answer is choice (D). When it started to infect humans, SARS-CoV-2 only elicited a primary immune response that was slow and allowed the virus to spread quickly.

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