What are antibodies and antibody tests? Does a negative antibody test mean the vaccine isn’t working?

What are antibodies?

An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. When our body is exposed to a pathogen for the first time, the immune system won't recognize the new antigen on its surface and our body will launch a response by producing antibodies that circulate in your blood.

These antibodies either kill the foreign pathogens or lock them onto their antigens, which are later neutralized by other immune cells.

In Sars-Cov-2 cases, antibodies are produced by cells called B lymphocytes as an immune response to the COVID-19 antigen. The role of these antibodies can be protective and as evidence to the exposure of SARS CoV-2 (COVID-19) or after vaccination. Vaccines teach your body to mimic this response without actually getting sick.

Antibodies are produced late in the first week (5-7 days). IgM antibody reaches peak by end of 2nd week (14 days). IgG antibody usually becomes positive after 8 - 13 days of infection & peaks around 21 days.

SARS-CoV-2 antibody (often referred to as serology) tests look for antibodies in a sample to determine if an individual has had a past infection with the virus or been vaccinated against.

What are antibody tests?

Different antibody tests like Total Antibody, IgG Antibody, which can be either qualitative or quantitative are available. Most of these tests detect antibodies to one of the two types of protein from coronavirus: Nucleocapsid (N) or Spike (S) protein.

Antibody testing identifies individuals who may have developed an immune response after infection with the virus. A positive test means either the person has been infected in the past or has been vaccinated. A negative result means antibodies to the virus were not detected which implies that the person has not been infected or vaccinated previously. It may also imply that the person has not generated sufficient immune response despite exposure to antigen.

Does a negative antibody test mean the vaccine isn’t working?

Though it is not recommended, most people would take antibody testing after vaccination to understand whether their body has generated an immune response to the virus which can help them to fight against the virus. The presence of antibodies would indicate an adequate immune response.

Usually, if testing is done after 2 weeks of the last vaccine dose, the person may know antibody presence and, in some tests, levels of these antibodies. The studies are still going on to understand what levels are required to neutralize the virus and protect individuals.

However, a vaccinated person can get a negative result from a serology test, even if the vaccine was successful and protective. First and foremost, The COVID-19 vaccine trains immune cells to notice and attack the signature spike protein. Any test which uses nucleocapsid protein instead of spike protein can show negative. The other reasons for such negative tests are: Testing too early after the vaccine, or failure of the immune system in an immunosuppressed person. People with compromised immune systems should still get vaccinated.

Experts say it would be rare to have no antibodies after receiving the vaccine.

 

(This article is attributed to Dr. Ramesh Kinha, Chief Pathologist, Medall Healthcare Pvt Ltd.)

(Disclaimer: The opinions expressed in this article are those of the author. They do not reflect the views of India TV)