The National Institutes of Health has called for 10,000 healthy volunteers to take part in antibody testing to determine undetected cases of COVID-19. So what is an antibody test? What does it measure and what does it mean?
Antibody tests are analyses of human blood samples which look for the presence of particular antibodies in the blood. They allow researchers and doctors to see the body’s past response to a disease. Antibodies are proteins produced by B cells of the immune system which bind to pathogens and enhance the immune system’s ability to eliminate an infection. B cells produce antibodies with structures designed to bind to one specific antigen once that antigen has been identified in the body. Therefore, the presence of antibodies of a particular structure indicates that the immune system has mounted a response to that antigen.
There are also different classes of antibodies, which have abbreviations such as IgM, IgG, IgD, and IgA, standing for immunoglobulin __. The first time you are infected with a particular pathogen, you will produce mostly IgM antibodies. Near the end of the first infection, the body begins to undergo “class-switching,” in which a different class of antibody better suited for fighting the infection is produced. Because of this, each following immune response is more refined and effective. For COVID-19 antibody tests, researchers are looking for anti-SARS-CoV-2 S protein IgG and IgM antibodies. IgM are generic antibodies, but IgG antibodies are typically best suited for viral infections. As Lab Tests Online indicates, the type of antibodies in the blood is a picture of a person’s infection history. If the blood sample reveals mostly IgM antibodies, the person has likely been infected only once and is not far out (less than a few weeks) from the initial infection. If there is a large concentration of both IgM and IgG antibodies, the person is likely a few weeks out from initial infection or may even be in the midst of a second infection. If the blood sample primarily contains IgG antibodies, the person has likely fought off the pathogen multiple times already. These people would be much less likely to transmit an infection to others; however, they may contribute to infection by cross-contaminating surfaces with a pathogen, even as they themselves are asymptomatic.
In a PBS article, doctors describe antibody tests as a critical response to this pandemic. One doctor explained that knowing whether he had been exposed and developed some immunity to the virus would help him feel more comfortable performing procedures like intubation that involve greater risk of exposure. Antibodies can also be used to passively immunize those fighting an infection. Researchers are already collecting antibody-containing blood plasma from those who have recovered from COVID-19 to potentially use in the treatment of severe cases. The novel coronavirus appears to have a low rate of reinfection, so antibodies may serve as proof of immunity, allowing people to return to normal life.
If NIH testing reveals that a large proportion of the population has been infected and recovered, we may be able to loosen social distancing orders sooner that expected. Italy and other countries are already doing antibody testing, and the WHO plans to start soon as well. In the meanwhile, though…
Hannah Biowell Blog 