The Biology of Vaccination

I was never too involved in the vaccination debate until I came to the United States. Back home in India, the majority of us seem to be grateful to science for being able to wipe out dreadful diseases like MMR (mumps-measles-rubella) and polio, and prevent the lifelong suffering of thousands of people. A friend recently mentioned that his mother refused to vaccinate him as a child when she observed escalated fever-like-symptoms every time he got an immunization shot. This is one small example of a widespread scientific ignorance that lures people into believing in absurd anti-vax propaganda.

Let’s talk about the biology of vaccination. A vaccine is a weakened form of a disease-causing agent that boosts the immune system and provides protection against natural infection. This “agent” may be an altered form of the infection or its less dangerous close relative. A vaccine is usually combined with an adjuvant – a chemical that enhances the immune response. Prior to vaccination, a process known as variolation remained popular in the 17th and 18th century. In this, scab material taken from a mild form of smallpox was inoculated through the skin to curb the disease. Variolation was in no way harmless and therefore ceased to be in use when safer alternatives were sought. The history of vaccination is one of the most interesting stories in the field of science and medicine. Edward Jenner (the father of Immunology) – after having observed that milkmaids exposed to cowpox were protected from smallpox disease,  treated the locals with cowpox scabs and successfully prevented the occurrence of smallpox.

So how does vaccination work? I have briefly talked about the two main kinds of immune responses in one of my earlier posts. Further, acquired immunity consists of antibody (humoral) response and cell-mediated response that involves various types of white blood cells (WBCs) like macrophages, dendritic cells, T-lymphocytes and B-lymphocytes. When an infectious agent enters the body, chemicals called chemokines and cytokines recruit WBCs to the area of infection. The pathogen is broken down into its constituent proteins by Antigen-Presenting Cells (APCs) and is then “presented” to the helper T-lymphocytes (CD4+ T cells). These lymphocytes actively mediate protective immunity.

In humoral immunity, the receptors on B-cells recognize specific antigenic proteins, get activated and multiply to make hundreds of identical cells. Upon maturation, these plasma cells release a large number of antibodies that are specific to the antigen. This rapid increase in the number of antibodies is sufficient to eliminate the pathogen. Apart from the B-cells, cytotoxic T-cells (CD8+ T cells) also induce an immune response by directly destroying antigens that are presented by the APCs.

Primary exposure to pathogen via vaccine and secondary exposure to pathogen via infection - Sequence of events.
Primary exposure to pathogen via vaccine and secondary exposure to pathogen via infection – Sequence of events with respect to humoral immunity. Cell-mediated immunity works similarly through cytotoxic T cells – Activated cytotoxic T cells directly destroys the antigen. (Not shown) — CLICK TO ENLARGE —

When the infection is cleared, the immune response reduces and so does the number of antibodies and cytotoxic T-cells. During this time, some of the T- and B-cells become memory cells and preserve their antigen-specific surface receptor. These cells stick around in our serum and wait for a subsequent attack by the same pathogen. This is the crux of vaccination.

When our body is invaded by the same pathogen again, these memory cells immediately proliferate and release surplus of specific antibodies against it. This secondary response is faster and involves a greater number of cells, and is therefore more effective than the primary response. Vaccination establishes a pool of memory cells that are specific to the antigen and prepares the body in case of future infection. Therefore, when a weakened form of the pathogen is intentionally administered to us, our body develops an “actively acquired immunity” for a quicker and a more efficient secondary response.

Antibody response during primary and secondary exposure
Antibody response during primary and secondary exposure

The milkmaids from Edward Jenner’s anecdote had acquired an active immunity for smallpox virus because they were previously infected by the cowpox virus (both poxviruses, members of the Poxviridae family) due to their occupation. Also, when my friends mother observed an escalated fever-like symptoms after the vaccine shot, it was merely the body’s primary immune response to the infection – completely normal and a sign of an actively functioning immune system.

Though the science of vaccination is pretty forthright, many concern arises regarding its safety, constituents, production and side-effects. It is important to understand that every immune system is unique due to which every person may respond differently to different vaccines. Many of the health and safety claims (with respect to autism, mercury, formaldehyde, and so on) have already been debunked extensively by reputed scientific sources. Also, parents choosing not to vaccinate their kids against the government’s decision are endangering the rest of the community. Herd immunity works when the larger part of the population is resistant to a pathogen providing protection to those without immunity thereby preventing an outbreak. And finally, if you’re against vaccination due to your religious beliefs, please pack up and leave.

Interestingness –

  1. How the anti-vaccine movement is endangering lives
  2. The dangerous consequences of anti-vaccine propaganda in one map
  3. Understanding Herd Immunity
  4. 9 vaccination myths busted. With science!

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