What is immunity?

• Immunity is your body’s ability to recognize germs to prevent them from causing illness. 
• The immune system’s job is to help identify and eliminate dangerous germs that enter the body before they can cause disease or damage. 

Types of immunity

There are two types of immunity: innate and adaptive.

Innate Immunity

• Innate immunity is the immune system that is present when you are born which is body’s first line of defense against germs. 
• It includes physical barriers, such as skin and mucous membranes, and special cells and proteins that can recognize and kill germs. 
• Innate immunity is like first aid — an immediate response, not strong enough to prevent pathology if the virus is highly virulent or the ‘inoculum’(infecting virus load) is heavy. 
• Innate immunity then passes the baton to adaptive immunity, which takes several days to develop and become effective.

Adaptive Immunity

• Adaptive immunity is protection that your body builds when it meets and remembers antigens in the body. 
• When your body recognizes antigens, it produces antibodies to fight the antigens. It takes about 14 days for your body to make antibodies. More importantly, the body memorizes this fight so that if its meets the same antigen again, it can recognize and attack more quickly. 
• Antibody production is one of the most important ways that immunity is developed.
• There are two types of adaptive immunity: active and passive.
  • Active Immunity – antibodies that develop in a person’s own immune system after the body is exposed to an antigen through a disease or when you get an immunization (i.e. a flu shot). This type of immunity lasts for a long time.
  • Passive Immunity – antibodies given to a person to prevent disease or to treat disease after the body is exposed to an antigen. Passive immunity is given from mother to child through the placenta before birth, and through breast milk after birth. It can also be given medically through blood products that contain antibodies, such as immune globulin. This type of immunity is fast acting but lasts only a few weeks or months.

Based on the cells involved in immunity

• • Immunity can be classified as 
    • B cell mediated or antibody mediated immunity and 

• T cell mediated immunity.
• Antibodies are protein molecules that recognise and bind to viral antigens. 
• Some among them tend to neutralise viruses from infecting fresh host cells. 
• Some viruses then adopt other mechanisms to infect host cells, and that is when T-cell immunity may come to the rescue. 
• In most viral infections, the presence of antibodies in the blood is sufficient to classify individuals as immune. 
• But unlike them, antibodies for COVID-19 wane fairly soon. 
• In persons with asymptomatic infections or mild COVID-19, nearly half will have no detectable antibodies after two months. 
• This phenomenon of short-lived antibodies and consequent re-infection is also seen in some other respiratory tract viruses. 
• Generally, re-infections are mild or asymptomatic, presumably due to protection afforded by T-cell immunity.

Does disappearance of antibodies for the COVID-19-causing virus mean that protection after one infection does not last? 

• Knowing that reinfection with symptoms has so far been proven in only about ten cases among millions infected, protective immunity after the first infection is probably durable. 
• The observed protection in the face of non-detectable antibodies highlights the need to study T-cell immunity.

More long-lasting

• In COVID-19 infection, T-cell immunity is more long-lasting than antibodies. 
• It resides in a subset of white blood cells called T-lymphocytes, or T cells. However, the test for assessing T-cell immunity is complicated and expensive. 
• Serial evaluation of T-cell immunity can help determine its durability after vaccination. 

Recent study findings from Karolinska Institute

• About 25% of blood donors in 2019, prior to the outbreak of COVID-19 infection in Sweden, had T-cell immunity against it. 
• This increased to 50% in 2020 after the pandemic had entered the country. 
• These observations imply that prior exposure to some other coronaviruses had evoked “cross-reacting” T-cell immunity towards the COVID-19 coronavirus. 
• In the same study, many contacts of proven COVID-19 patients had T-cell immunity, even though antibodies were undetectable. 
• This indicates that in those exposed to the COVID-19 virus, T-cell immunity occurs even without a detectable antibody response.

How T cell mediated immunity is long lasting?

• The Karolinska investigators found that the immune T cells had ‘stem-cell’ like characteristics indicating their long-term survival and potential of quick multiplication. 

Significance of T-cell immunity

• T-cell immunity is a better and more durable marker than antibodies of past infection for this novel virus. 
• Those with T-cell immunity may need no vaccine, or only a single dose of a two-dose vaccine regimen.

Conclusion

• If India’s vaccination policy, when made, recommends that vaccines may be conserved for priority use for non-immune subjects, then, a rapid T-cell immunity test will be better than antibody tests. 
• Therefore, developing simple and rapid assays for T-cell immunity should be a priority for Indian scientists to work on, quickly. 

Reference:

1. https://www.thehindu.com/todays-paper/tp-opinion/t-cell-immunity-and-covid-19/article33121358.ece