Vaccines have been monumental in combating infectious diseases since their inception. They leverage the immune system’s innate ability to learn and adapt to counteract pathogenic threats and prevent subsequent infections. Vaccines have not only revolutionized medicine but have significantly shaped the course of human history through their profound impact on public health.

Vaccines work by mimicking an infection, thereby teaching the immune system to identify and combat the actual disease. They stimulate a response from the immune system without causing the disease or its associated complications (1). Various types of vaccines have been developed, including live-attenuated vaccines, inactivated vaccines, subunit, recombinant, polysaccharide, and conjugate vaccines, toxoid vaccines, and more recently, nucleic acid vaccines (2).

Mechanism of Action of Vaccines

When a pathogen invades the human body, it triggers an immune response. This response begins when certain cells of the immune system, known as antigen-presenting cells (APCs), ingest the pathogen and present fragments of it, called antigens, on their surface. These APCs, which include macrophages, dendritic cells, and B cells, then migrate to the lymph nodes where they present these antigens to T cells, a type of white blood cell (3).

The interaction between the APCs and the T cells stimulates the T cells to proliferate and differentiate into two main types: cytotoxic T cells, which can directly kill infected cells, and helper T cells, which aid in activating B cells. When a B cell encounters the antigen that matches its receptor, with the help of helper T cells, it becomes activated and starts to proliferate. Some of these B cells differentiate into plasma cells, which produce antibodies specific to the antigen. These antibodies can then neutralize the pathogen, preventing it from infecting cells (4).

Memory cells are also produced during this process – both memory B cells and memory T cells. These cells persist long after the initial response has cleared the pathogen. If the person is exposed to the same pathogen again, these memory cells enable a quicker and more potent immune response, known as the secondary immune response (5).

Vaccines exploit this natural mechanism of the immune system. They introduce antigens or attenuated (weakened) or killed pathogens into the body, triggering an immune response. The immune response to a vaccine is similar to the one produced by a natural infection but without causing the disease or its associated risks. This immune response results in the production of memory cells and antibodies specific to the antigen, priming the immune system to respond more effectively if it encounters the same pathogen in the future (6).

Significance of Vaccines

Vaccines have a significant role in preventing diseases, reducing morbidity and mortality rates worldwide. For instance, the introduction of the measles vaccine in 1963 led to a 99% decrease in measles cases in the United States (7). Globally, vaccines save 2-3 million lives each year and protect many more from the debilitating effects of diseases like pneumonia, diarrhea, and influenza (8).

Vaccines also play a crucial role in achieving herd immunity. Herd immunity occurs when a significant proportion of a population becomes immune to an infectious disease, reducing its spread. For some diseases, herd immunity can effectively stop disease transmission, protecting even those individuals who cannot be vaccinated due to medical reasons (9).

In conclusion, vaccines represent one of the most effective public health interventions in history. They work by stimulating the body’s immune system to recognize and fight off specific pathogens. By doing so, they prepare the immune system for potential future encounters with the pathogen, thereby preventing disease. As we continue to face new infectious disease threats, the development of new and more effective vaccines will continue to be a priority in medical research.

References

Plotkin S. (2014). History of vaccination. Proceedings of the National Academy of Sciences, 111(34), 12283–12287. https://doi.org/10.1073/pnas.1400472111

Centers for Disease Control and Prevention. (2018). Types of Vaccines. https://www.cdc.gov/vaccines/vpd/vpd-vac-basics.html

Abbas, A. K., Lichtman, A. H., & Pillai, S. (2018). Cellular and Molecular Immunology (9th ed.). Elsevier.

Murphy K., Weaver C. (2016). Janeway’s Immunobiology (9th ed.). Garland Science.

Hammarlund E, Lewis MW, Hansen SG, et al. (2003). Duration of antiviral immunity after smallpox vaccination. Nature Medicine, 9, 1131–1137.

Poland GA, Ovsyannikova IG, Kennedy RB. (2014). Personalized vaccinology: A review. Vaccine, 36(36), 5350-5357. doi: 10.1016/j.vaccine.2018.07.029.

Centers for Disease Control and Prevention. (2015). Measles history. https://www.cdc.gov/measles/about/history.html

World Health Organization. (2018). Immunization coverage. https://www.who.int/news-room/fact-sheets/detail/immunization-coverage

Fine P, Eames K, Heymann DL. (2011). “Herd immunity”: a rough guide. Clinical Infectious Diseases, 52(7), 911-916. doi: 10.1093/cid/cir007.

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