When a 27-year-old pregnant woman who was already infected with the virus got an antibody test, it showed she had very few antibodies against COVID-19. The same thing happened with a 49-year-old man who had received one shot of the vaccine. While studies have shown those infected, or who received the vaccine have higher levels of antibodies, that may not always be the case.
There are now questions on whether they are more vulnerable to getting reinfected by SARS-CoV-2. Meanwhile, a Lucknow man who did not develop antibodies a month after his first dose even filed a case against Serum Institute of India, India Today reported.
Research has stated that an antibody response even though the easiest to detect for detecting an immune response is not the only response elicited by the body when fighting SARS-CoV-2. While the natural defenses of the infected cells already start fighting the virus, they also signal specific immune system cells to get activated.
The pregnant woman, who chose to stay anonymous, said that her gynecologist asked her not to be worried and continue taking all the necessary precautions. Her doctor had told her that even though she did not have antibodies, her body had different mechanisms to produce an immune response if she would encounter the virus again.
For Tanmaya Nanda, an antibody test a month after receiving his first shot of Covishield showed that his antibody titer was below the requisite 1. He is yet to consult a doctor to understand if he is more vulnerable to the disease or has developed other immune responses to the virus.
In a recent study published in Nature, scientists showed that antibodies against SARS-CoV-2 were present in a person's body up to a period of 12 months after infection. The presence of memory B cells in the bone marrow that lead to the formation of antibodies is only one type of immune response that the body generates.
BOOM spoke to Dr. Satyajit Rath, immunologist, IISER Pune, to understand the different aspects of immunity that come into play while fighting the impact of SARS-CoV-2.
"There are indeed reports of some COVID-19-recovered patients having relatively low levels of SARS-CoV-2 antibodies. There are also reports of some volunteers in vaccine trials showing relatively low vaccine-generated antibody levels. Both instances seem to be occasional, rather than being the norm," Rath explained.
Role Of T Cells In Immunity Against SARS-CoV-2
T cells are another form of immunity-producing cells in the body that do not lead to the creation of antibodies. They are activated whenever the body encounters a pathogen and a particular antigen that the cell dormant in the bone marrow can act against.
T cells are further divided into CD4 helper T cells and CD8 killer- cytotoxic T cells. The CD4 helper T cells send signals to the CD8 T cells as soon as they come across a foreign body. The CD8 cells are then responsible for killing these foreign particles.
Explaining the role of T cells against SARS-CoV-2, Rath emphasised that the level of protection provided by CD8 T cells against SARS-CoV-2 is still unknown.
"Antibodies arise from the responses of B lymphocytes (to infection or to vaccine). However, for generating high levels of high-potency antibodies as well as generating 'memory' B cells, these B lymphocyte responses require help from ongoing CD4 T lymphocyte responses, which are directed at either the infection or the vaccine. This is one way in which T cells provide useful immunity against SARS-CoV-2," explained Rath.
"Another group of T lymphocytes, the CD8 T cells, can also be activated, either by infection or by some vaccines. The mRNA vaccines such as NIH-Modern or BioNTech-Pfizer, adenoviral vaccines such as Covishield or Sputnik, and DNA vaccines activate them but they are not activated by subunit vaccines (Novavax) or inactivated whole-virus vaccines (Covaxin). Virus-targeted CD8 T cells can recognise and kill virus-infected cells in the body and since the virus grows in infected cells, virus growth is 'aborted' if the infected cell dies too soon," summed up Rath explaining the role of T cells.
Rath also explained how vaccines boost immune response in people who were already infected by the virus. "Any repetition of exposure of the body to these viral targets, either via an infection again or by vaccination, will re-stimulate 'memory' B cells and CD4 T cells, and together they will generate even higher levels of high-potency antibodies. This is what we commonly refer to as a 'boost' effect."
Does The Body Have Any Other Immune Responses?
Along with B cell and T cell responses, the body also has natural ways of reacting instantly to a foreign particle. While B cell and T cell induced immunity falls under adaptive immunity as the body adapts to infection, innate immunity is the first line of defense of the body to any pathogen or tumorous cells. This includes physical barriers such as skin, the gastrointestinal tract, the respiratory tract, the nasopharynx, cilia, eyelashes, and other body hair as well as other defense mechanisms such as secretions, mucous, bile, gastric acid, saliva, tears, and sweat.
In the case of COVID-19, the respiratory tract tries to fight SARS-CoV-2 with its own defense mechanisms as the response of B cells and T cells is still slowly building up.
The doctor explained that for SARS-CoV-2, the virus replicates somewhere in the respiratory passages-- nose, throat, and lung airways, where a droplet from the breathed air, laden with virus particles, has landed. A virus particle, he said, will stick closely to a cell of the respiratory passage lining, and its genetic material will get into that cell. It will then take over the biochemical machinery of the cell and use it to make virus copies, assemble them and let them out of the cell to infect nearby cells. However, the cell that gets infected also has virus sensors that get activated when virus genetic material comes in.
Explaining the immune response process, Rath said that these virus sensors activate antiviral mechanisms (type 1 interferons, for example). This then reduces virus growth and inflammatory mechanisms that recruit cells of the immune system to the infected area and restrict the infection to that area. "A micro-containment zone," Dr Rath said. "All of these early immune responses and mechanisms are innate," he added.
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