What are the symptoms of low immunity
Antibodies - what exactly are they and how can they help against the coronavirus?
Immunity to Disease
The human body has a variety of mechanisms to react to pathogens (e.g. bacteria, parasites or viruses). First of all, the skin and mucous membranes provide a natural protective barrier. The various body secretions such as saliva or gastric juice contain substances that can kill pathogens that have overcome this first barrier. These mechanisms are part of the so-called unspecific immune response. In addition, the body has the ability to defend itself against very specific pathogens - with the specific immune defense. Antibodies are a key building block for this.
What are antibodies
antibody (also called immunoglobulins, abbreviation: Ig) are proteins that are made by the body after cells of the immune system have discovered the pathogens in the body. These antibodies are tailored to the particular pathogen and can attach themselves to the surface like magnets. The pathogens “marked” in this way can move more poorly and are more easily recognized by the immune system. This helps eliminate the pathogens before they can multiply and damage the body further.
When can antibodies be detected in the blood by means of a test?
The first antibodies are usually detectable in the blood from about a week and a half after the pathogen has entered the body. They are divided into different classes (IgM, IgE, IgG, IgA, IgD). The different classes are formed at different speeds and differ in their function and effectiveness. IgM antibodies are formed the fastest in an infection. They are the antibodies for the "gross" and are replaced by IgG antibodies in the course of the disease. These have a more specific effect, can be produced in larger numbers by the body and are present in the blood for a long time. In this way, the body can react quickly to a renewed infection and therefore often without renewed symptoms of the disease.
Antibody diagnostics for coronavirus infections
Antibodies after a coronavirus infection
Basically, it can be assumed that the immune system is well known to the pathogen after an infection and that specific antibodies are present in the blood. As soon as these are detectable, one can assume that an infection has taken place and that there is a certain immunity. Although the antibodies in the blood can disappear from infections that have occurred long ago, the lack of detection of antibodies in the blood does not necessarily mean that there is no longer any immunity to the pathogen. Rather, our body has memory cells which, in the event of renewed contact with the pathogen, can immediately and quickly stimulate the production of antibodies.
For some time now, antibody tests have also been approved for the novel coronavirus. As these tests take time to produce, the number of tests available is currently limited, but is increasing from week to week. There are currently two considerations as to how the new antibody tests can be used: The most frequently discussed use is as a so-called “screening instrument” to find out what percentage of the population has already passed through the infection. It is hoped that this will provide better information about the course and severity of the pandemic and, in particular, about the number of unreported cases. In other words, those people who are sick with the virus but have only developed mild symptoms or no symptoms at all). These examinations are currently not carried out on all citizens, but as part of studies.
Antibody studies in coronavirus
For example, the Robert Koch Institute (RKI) is currently carrying out three large antibody studies in areas particularly affected by Sars-CoV-2, in randomly selected people from the general population and in blood donors.  Such “cross-sectional examinations” are intended to determine how many people have already been infected with the new type of coronavirus. The fact that people of different age groups and people from different parts of Germany are examined also makes it easier to understand which age groups and which localities are particularly affected. The second scenario is indirect evidence of an infection taking place. The time of contact with the virus cannot be determined by means of a single test - a problem with the so-called "rapid tests" (see below) - but with the "first" SARS coronavirus, for example, there is the possibility of a significant increase in the antibody concentration in two consecutive measurements to indirectly detect an acute infection.
Antibody detection test for COVID-19
In contrast to the direct detection of a pathogen, such as the coronavirus from a throat swab, the antibodies can only be detected in a blood sample. Coronavirus antibodies are detected in the test tube, where the pathogen-specific antibodies obtained from the blood bind to artificial proteins that look like the surface of the coronavirus. Such a procedure is complicated and requires several steps which are normally carried out in a laboratory. In the end, the result is then positive or negative. A positive result means that antibodies have been formed against the pathogen and that there is at least some immunity. The findings from the laboratory are usually very meaningful. The same method is also used in so-called “Corona rapid tests”. However, due to the greatly simplified work steps in the small test kits, the results are more often incorrect and therefore have to be assessed critically. The name is also misleading here, because no direct pathogen detection can be made. The tests are usually only certified by the manufacturer itself; there is no validation in direct comparison with existing laboratory tests.
Antibody test strengths
Should the quality of the rapid antibody tests increase in the future, a broader range of applications would be conceivable:
In addition to being used as a screening instrument for examinations at the population level, medical staff could, for example, be specifically examined and staff with coronavirus antibodies could be used specifically for COVID-19 patients or risk groups due to immunity.
There are currently debates about whether people with antibodies should receive an “immunity pass” that confirms the antibodies against the novel coronavirus and allows more differentiated measures in the event of current or future contact and work restrictions. Several initiatives are currently working on a technical basis for such a “passport”. Whether such a passport can lead to discrimination, which additional data would have to be stored there and how long an immunity actually lasts is currently still open.
Antibody plasma donation for therapy
The identification of healthy people who already have antibodies against the novel coronavirus could help in the treatment of patients seriously ill with COVID-19. Because these Sars-CoV-2 antibodies can be specifically filtered out of the blood of the recovered people and given as therapy to patients suffering from COVID-19. This process is complicated, however, and studies on this are still at a very early stage.
When are coronavirus antibodies undetectable?
Symptoms of illness from other pathogens
The most common symptoms of a SARS-CoV-2 infection are, according to the surveys of the RKI, cough, fever and runny nose and therefore very unspecific.  A negative test result can therefore always indicate another illness such as the annual flu.
Tested too early
Since the corresponding antibodies only appear a few days after infection, there is a risk that you test “too early” and thus the test will be negative even though you are already infected.
No more antibodies detectable
As a rule, the amount of antibodies decreases over time if there is no renewed contact with the pathogen. The amount of antibodies can be reduced so much that they can no longer be detected in a laboratory test. However, no evidence does not necessarily mean that there is no longer any immunity. Rather, we know that our immune system has a memory and can produce new antibodies again if a disease occurs again.
There are disorders of the immune system that make the formation of antibodies difficult or even impossible. These types of disorders can, for example, be genetic or caused by medical measures (medication, radiation, etc.). This means that it is sometimes not possible for those affected to develop immunity.
Is it possible to re-infect with the coronavirus?
According to the current state of knowledge, it is still unclear to what extent the presence of antibodies actually means immunity. In particular, based on a study from South Korea, which reported reinfections among those already infected with SARS-CoV-2, doubts were raised about the immunity after infection.  However, many virologists doubt that this was actually a re-infection and suspect that the result of the study was due to weaknesses in the test method. It can happen that at the end of the illness only a small amount of viruses are still present in the throat. As a result, no viruses would sometimes be found on the cotton swabs in the case of throat swabs. If viruses happen to be found elsewhere in a later smear, this can be incorrectly interpreted as reinfection.
Until the opposite is proven, however, the result of the South Korean study is conceivable. In order to find answers here quickly, the World Health Organization (WHO) got involved and checked the results.
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