Daily Answer Writing
27 April 2021

QUE) Discuss the types of immunities in the human body. Illuminate the factors on which herd immunity depends as well as the means to achieve it. (150 Words)

Source <https://indianexpress.com/article/explained/complexities-of-herd-immunity-coronavirus-second-wave-7290409/>

GS 3: Science and Technology

Approach Answer:

Introduction: Immunity can be defined as a complex biological system endowed with the capacity to recognize and tolerate whatever belongs to the self, and to recognize and reject what is foreign (Antigens). This ability to distinguish different substances, called specificity and this ability to respond more vigorously to repeated exposures to the same microbe, known as memory.


These immunities can be broadly of two types

              1. Innate immunity (also called natural or native immunity): It provides the early line of defense against microbes. It consists of cellular and biochemical defense mechanisms that are in place even before infection and are poised to respond rapidly to infections.

              2. Adaptive immunity (also called specific or acquired immunity): occurs after exposure to an antigen either from a pathogen or a vaccination. It recognizes and reacts to a large number of microbial and non-microbial substances. The unique components of adaptive immunity are cells called lymphocytes which has two arms: —

                             i. Antibodies are protein molecules that recognise and bind to viral antigens. Some among them tend to neutralise viruses from infecting fresh host cells.

                             ii. Some viruses then adopt other mechanisms to infect host cells, and that is when T-cell immunity may come to the rescue.


The Adaptive immunity can further be classified as given in the following diagram.




Herd Immunity is a form of indirect protection from infectious disease that occurs when a large percentage of a population has become immune to an infection, whether through vaccination or previous infections. This percentage differs for different diseases depending on their degree of infectiousness.


Factors determining herd immunity

              1. Natural infections: The antibodies produced due to natural spread of an infectious disease. This can be measured through a sero-prevalance survey.

              2. Vaccinations: It leads to artificial adaptive immunity. A vaccine generally contains an agent(Antigen) that resembles a disease-causing microorganism and is often made from weakened(LIVE) or killed(INACTIVATED) form of microbe, its toxins, or one of its surface protein. This generate immune response.

              3. R-naught or infectiousness of a disease:'R-naught' or Ro tells how many people can an infected person infect or the transitiveness of a disease. Greater the Ro, greater the anti-body prevalence is required in the population to achieve herd immunity. Many variants of Corona-virus have R-naught rate of more 4.5; One person can infect 4.5 persons. In such a case herd immunity comes a larger (78%) of population is immune.


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.


In such a case, there are only two means to achieve herd immunity are:

              i. Spread of the disease

              ii. Mass-vaccination drives.


Conclusion: The herd immunity achieved by wild spread of a disease does a great harm, by over-straining the health infrastructure of a country. The only viable way, thus, to stop an infectious disease is a mass vaccination drive. For this rapid production and addressing vaccine hesitancy are only two means against an infectious disease like COVID-19.

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