What is important to know about antibodies to coronavirus and tests for them

What are antibodies to coronavirus

AntibodiesAntibody (Serology) Testing for COVID-19: Information for Patients and Consumers / FDA (or immunoglobulins) are special proteins that the immune system produces to fight off viruses, bacteria, and other pathogens that enter the body.

For each infectious disease, the body produces its own antibodies. Immunoglobulins, designed to fight the flu, cannot fight adenovirus or, say, hepatitis.

To fight SARS ‑ CoV ‑ 2, our body also produces very specific immunoglobulins. They are called antibodies to coronavirus.

What are antibodies to coronavirus

In general, antibodies are divided into five typesAntibody Structure, Function, Classes and Formats / SinoBiological. They can be very schematically described as follows.

• IgA, or immunoglobulins A. This is the body's first line of defense. They respond to antigens This is the name for substances that the body considers foreign, whether they be viruses, bacteria, fungi or their parts.
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  and bind them to prevent penetration into body tissues. IgA is usually abundant on mucous membranes.
• IgE (immunoglobulins E). It is believed that their main function is to protect the body from parasites.
• IgD (immunoglobulins D). Science has not yet fully figured out their purpose. But it is known that they are present on the surface of B-cells - lymphocytes, which, if necessary, are able to produce any type of antibodies. IgD is possibly starting this process.
• IgM (immunoglobulins M). Immunity, faced with a virus or bacterium that has entered the body, first of all begins to produce IgM. These are the most complexly organized antibodies, a kind of special forces that aggressively attacks pathogens.
• IgG (immunoglobulins G). If IgM is special forces, then IgG is patrolHad COVID? You’ll probably make antibodies for a lifetime / Nature. Antibodies of this type provide long-term protection. They can be in the blood for months, years, and even decades after an illness. So that, if the body suddenly encounters an infection again, take the first blow and at the same time start the accelerated production of IgM.

How the immune response works in COVID-19 is not yet clearInterim Guidelines for COVID-19 Antibody Testing / CDC. But it is already known that IgM and IgG play the most important role in it. It's believed that:

• IgM Is a sign of an acute infectionHongyan Hou, Ting Wang, Bo Zhang, Ying Luo, Lie Mao, Feng Wang, Shiji Wu, and Ziyong Sun. Detection of IgM and IgG antibodies in patients with coronavirus disease 2019 / Clinical & Translational Immunology. If they are, then the person is sick with COVID-19 right now. Typically, this type of antibody peaks about two weeks after infection. And then it quickly drops to almost zero, when the person finally recovers.
• IgG talk about the formed long-term immunity. Antibodies of this type arise almost simultaneouslyInterim Guidelines for COVID-19 Antibody Testing / CDC with IgM, however, they do not disintegrate after a couple of weeks, but persist for at least several months after infection.

It is these proteins that doctors look for when they do tests for antibodies to coronavirus.

What are the tests for antibodies to coronavirus and what their results mean

A blood sample is taken for analysis. Depending on where the blood was taken from and how it is examined, tests are divided into several types.

Qualitative and quantitative

• Qualitative. Such tests determine if there are coronavirus immunoglobulins in the blood in principle. There are only two possible results: antibodies are detected (that is, the body has already encountered an infection) - and antibodies are not detected (most likely, you have not yet been sick. However, not a fact - the details are below).
• Quantitative. This is a more complex analysis. It allows you to find out how many IgM and IgG antibodies are contained in a unit volume of blood serum. Based on this, it can be assumed whether a person has COVID-19, at what stage the disease is and, in part, how strong the immunity developed to coronavirus is. However, the last point is controversial: science still cannot say for sureCOVID-19: serology, antibodies and immunity / WHOwhether a high level of antibodies indicates an equally high antiviral protection.

ELISA and rapid tests

If a laboratory assistant takes blood from a vein, it means that you are given an ELISA test.Coronavirus infection: is there or not? / Moscow City Health Department. The enzyme-linked immunosorbent assay (this is how the abbreviation ELISA stands for) has high accuracy and is most often quantitative: it helps to establish how much IgM and IgG is contained in the blood.

Each laboratory has its own reference values ​​for antibody levels - they must be indicated on the analysis forms. But if you do an ELISA test through a state polyclinic, experts from the Moscow Department of Health suggest interpreting the results as follows:

• IgM <1, IgG <10. You don't have antibodies to coronavirus.
• IgM - from 1 to 2, IgG <10. Questionable result. You do not have long-term antibodies to coronavirus, but a slightly elevated level of immunoglobulin M suggests that the virus can multiply in your body right now. That is, you may be sick with COVID-19, but carry it asymptomatically. Doctors recommend in this case to limit contact with people for a week, and then repeat the test.
• IgM> 2, IgG <10. You have COVID-19. Even if you don't feel it. You need to isolate yourself so as not to infect others.
• IgM> 2, IgG> 10. Another questionable result. In the sense that a high level of long-term IgG antibodies means that you have already been ill and, most likely, have developed immunity. However, elevated IgM levels indicate that the virus may still be in your body and that you are capable of infecting others. Protect them: limit contacts and maintain social distance for at least 7 days. Then do ELISA again.
• IgM <2, IgG> 10. You have been ill and have developed long-term antibodies to the coronavirus. Chances are, you are no longer contagious. But in any case, do not neglect the safety rules: keep a social distance, wash your hands regularly and wear a mask in public places.

If for analysis it is enough to take blood from a finger (capillary), this is an express testOn the types of studies for the new coronavirus infection COVID-19 / Rospotrebnadzor. It is much simpler than ELISA and faster: its results will be ready in 15–20 minutes. However, the accuracy of rapid tests is significantly lower than in the analysis of venous blood. In addition, usually express analyzes are of high quality, that is, they do not provide information on the amount of immunoglobulins G and M.

IgG tests for the spike S-protein and other proteins of the coronavirus

In laboratory testing, IgG can also be determined to various proteins (antigens) of the coronavirus. This may provide additional information about immunity to COVID-19.

SARS ‑ CoV ‑ 2 contains several proteins, but only two of them are of diagnostic value - those that cause the most activeInterim Guidelines for COVID-19 Antibody Testing / CDC immune response.

• S is a spike protein that helps the virus attach itself to the cells of the body. Spike S-proteins make up the "spikes" of the coronavirus. Most of the first vaccines, including Pfizer, Moderna, AstraZeneca and domesticVaccination / InterFax "Sputnik V" and "EpiVacCorona" teach the body to produce antibodies to this particular type of protein.
• N - nucleocapsid protein. It protects the RNA of the virus and participates in the formation of new viral particles.

There is one more important point. The spike protein S contains two constituents - S1 and S2. In turn, S1 contains the so-called RBD-domain (receptor-binding domain) - a structure due to which the coronavirus binds to the receptors of the cells of the human body and is able to penetrate into them.

Based on this, the laboratory may offer you such antibody tests.

Spike (S) Protein IgG Test

If such antibodies are found, this means that the body has learned to attack the spines of the coronavirus so that it cannot dock with the cell. That is, he developed immunity to it after a previous illness or vaccination.

It should be remembered that IgGs are not produced immediately, so it is advisable to carry out such an analysis no earlier than two to three weeks after vaccination or the appearance of the first symptoms of COVID-19. If the spike protein test is done earlier, it may turn out to be false negative.

IgG test for RBD ‑ domain of spike protein

This is a variation on the previous analysis. Its results mean the same: if IgG is detected, then you have developed immunity after illness or vaccination.

Nucleocapsid (N) Protein IgG Test

But this analysis has a peculiarity. Antibodies to the N-protein appear only after a person has transferred COVID-19 naturally. After vaccination, there will be no such IgG.

That is, if you were vaccinated, and then decided to get tested for antibodies to coronavirus and chose this particular test, it will not show anything.

If you want to check if there is an immune response after vaccination, order a test for S ‑ protein, not N.

Is it worth taking tests for antibodies to coronavirus

Actually, this is a rather meaningless event. For you as a specific person. For several reasons.

1. Antibody tests will not tell you exactly if you are infected

US Food and Drug Administration (FDA) warnsAntibody (Serology) Testing for COVID-19: Information for Patients and Consumers / FDA:

• Even if you are infected, the antibody test may be negative. This happens, for example, when you do the test too early and your body has not yet produced enough immunoglobulins. It is most reliable to conduct a study no earlier than two weeks after infection. But by this time it loses its meaning: your body has already coped with the infection, or COVID-19 will show itself with obvious symptoms.
• Even if you are healthy, an antibody test may be positive.. Sometimes the analysis fails and shows immunoglobulins developed to a different, harmless type coronavirus infections - those that cause the common cold. This means that in fact you do not have COVID-19, but according to the test you do.

If you need an accurate diagnosis, go through PCR testing. An antibody test can only be used as an auxiliary factor - when doctors cannot figure out whether you are sick in any way, and are looking for additional pros and cons for each of the possible options.

2. They will not help determine if you are immune to coronavirus.

Even if the test detects antibodies, this does not mean that you are not at risk of COVID-19. Evidence-based medicine does not yet know what level of antibodies is needed for full protection and whether immunoglobulins are effective against SARS ‑ CoV ‑ 2 in principle.

3. Tests will not tell you for sure if you have had COVID-19 before

At first glance, everything is simple: if you have been ill, antibodies will be found in you, if you have not been ill, they will not be. But it is not so.

If the analysis shows the absence of antibodies, this can indicate three fundamentally different situations:

1. You haven't really had COVID-19.
2. You have had COVID-19 but have not developed antibodies. By the way, this happens with about every tenthInterim Guidelines for COVID-19 Antibody Testing / CDC infected. Moreover, the absence of antibodies does not mean that you are at risk. Other mechanisms that scientists have not yet figured out can provide protection against COVID-19, such as cellular immunity. Cellular immunity is based on the work of one of the types of lymphocytes - T-cells. .
3. You have a false negative result. Even quality tests sometimes fail and do not detect antibodies, although they are available.

Which of the options is yours, no one will say for sure.

4. They won't help you figure out if you need a vaccine.

Because, again, a high antibody titer is not yet a guarantee of protection against COVID-19. And zero does not mean that you have no immunity.

For this reason, the American Centers for Disease Prevention and Control (CDC) does not recommend testing immunoglobulin levels before inoculation. For it is pointless.

Does this mean that the test for antibodies to coronavirus does not need to be taken at all?

And here there are nuances. For a specific person, there is almost no practical value in such a study. But for science in general, bulk antibody testing is very useful.

Experts from the British Ministry of Health directly explainCoronavirus (COVID-19): antibody testing / Department of Health & Social Carewhy tests for immunoglobulins are needed. There are only two answers:

1. Testing allows you to roughly estimate how many people have already had a coronavirus infection.
2. Helps to track the spread of the virus throughout the country and within different social groups.

This is really important information. Thanks to her, scientists will better understand the features of the virus. Learn to more accurately identify risk groups. They will be able to predict and respond in a timely manner to the threat of another epidemic outbreak in a particular region.

But whether you need to participate in this is up to you.