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COVID-19 antibodies and your immune system

As the world celebrates vaccines during world immunization week (April 24-30) while it simultaneously fights a global pandemic, we’d like to relate what is known about disease immunity to what we know about the novel coronavirus “COVID-19.” As with all of our communication at doc.ai, our goal is to give you accurate information and clarify any misconceptions so that you can respond to the COVID-19 pandemic in a way that is consistent with the science and your values.


To start off, let’s explore the way the immune system works to protect us. The immune system is complex and responds differently to each virus it is exposed to. That’s because when your immune system first detects an infection, it develops multiple responses. The first response, known as the innate immune response, is when cells initially detect a virus. They produce a type of molecule (cytokines) to tell nearby cells that there’s a virus among them. As a result, they start making antiviral proteins to halt the invasion.


However, viruses can evade the body’s innate immune response. To recruit more help, a second response is deployed. Known as the adaptive immune response, the body activates a specific type of white blood cell called a helper T cell. Helper T cells can function as generals in your body’s fight against an infection. They coordinate the immune system’s response, including instructing another type of white blood cell called a B cell. B cells are similar to frontline soldiers; they are in the trenches fighting the infection. At the instruction of helper T cells, B cells multiply so that there are a lot of soldiers to fight the infection. B cells produce antibodies, which are like bullets that are designed to target a specific infection.


However, the goal of your immune system isn’t just to fight a specific infection your body is experiencing. It also provides a faster response to the same infection if it encounters it again. B cells accomplish these two tasks by producing different types of antibodies, also known as immunoglobulin. Immunoglobulin M (IgM) antibodies are the first type your body develops in response to an infection, followed by immunoglobulin G (IgG), which can last longer than IgM. Usually, the IgG antibodies stay in your body for a very long time so that your B cells can use IgG “bullets” against the same infection if you’re exposed to it later.


With this fundamental understanding of our immune response, you can understand the difference between determining whether somebody has COVID-19 and whether they were, at some point, exposed to SARS-CoV-2, the virus that causes COVID-19.


Testing for COVID-19 currently can be done in different ways, but the most common is to collect a sample via a nasopharyngeal swab and test for the presence of SARS-CoV-2 genetic material. We would typically expect to find SARS-CoV-2 genetic material in a nasopharyngeal swab if an individual were actively fighting a SARS-CoV-2 infection (whether or not they were symptomatic at the time). This means if we were to test a healthy patient with a nasopharyngeal swab for evidence of SARS-CoV-19 genetic material because he said he thought he had COVID-19 three weeks ago, the test would most likely be negative, even if he did have the virus three weeks ago. So, how can we determine if the individual is immune to a virus?


For most infections, including SARS-CoV-2, you can test an individual for the presence of IgG and IgM antibodies to the infection. However, antibody testing (sometimes called serology tests) is not perfect. Below are some of the challenges with antibody testing:

  1. In the early stages of an individual’s infection, they may have insufficient IgM antibodies, and they would not be expected to have any IgG antibodies for a test to identify antibodies. Therefore, an individual who has COVID-19 might test negative for SARS-CoV-2 antibodies.

  2. It’s possible for a person to have been infected by a different virus in the same ‘family’ and for the past infection to create false-positive results in an antibody test. For example, an individual who has been infected by a coronavirus that is different from SARS-CoV-2 might test positive for SARS-CoV-2 antibodies, even if they have not been exposed to SARS-CoV-2. This is called cross-reactivity.

  3. It is not known how long SARS-CoV-2 antibodies will remain present after a person no longer has COVID-19. Ideally, our bodies maintain IgG antibodies for months or even years. But, we don’t know if that’s true for SARS-CoV-2 antibodies. Decisions, such as which healthcare workers can return to work, can only be made on the basis of accurate antibody test results and a better understanding of how much immunity these antibodies confer over time.


What does the future hold for diagnosis of COVID-19 and identifying individuals who have immunity to the disease? It’s still early and there’s a lot we don’t know, but immunologists, scientists and the broader medical community are partnering together to research to validate these tests. doc.ai will have more to share around our efforts with this in the coming weeks so stay tuned.


Lastly, remember that if you do develop symptoms of COVID-19 — fever, cough, shortness of breath — call your doctor immediately. To help you monitor your disease, doc.ai has developed a COVID-19 Symptom Tracker. It was designed to help you keep tabs on your health and share that information with your healthcare team. While there is still no vaccine for COVID-19, take heart that we do have a growing body of research and new, innovative tools to help manage our current situation.


Stay well everyone.



- Dr. Chethan Sarabu, MD, director of clinical informatics at doc.ai


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