Vaccines as ninja trainers

Imagine entering a strange room filled with priceless sculptures blindfolded and with your ears plugged.  You’d stagger around and bump into a lot of things, maybe break some of them before you figured out where you were and how to navigate the unfamiliar space.  The art curators would be extremely unhappy with you and your street cred would be blown.  Now imagine entering the same room with your eyes wide open and ears alert as every sculpture is bathed in its own brilliant spotlight.  You’d know exactly which sculpture was which and, unless you are extraordinarily clumsy or had had three too many flutes of champagne at the art opening, you’d probably not destroy anything.

That is the difference between your immune system encountering a pathogen on its own and encountering it after you’ve been vaccinated against it.  Without a vaccine, your immune system is going to be slow and clumsy and could well cause more damage than can ever be repaired.  This was commonly the case in polio and whooping cough before vaccines against them became available.  Many children were permanently disabled and too many died.  With the vaccine, your immune system knows what to look for because it has seen similar things before and as such it would have to work much harder to cause the same degree of catastrophe.

Your immune system without vaccines (left) and after vaccination (right).

At a molecular level, vaccines and pathogens aren’t all that different, at least not in the eyes of the immune system[1].  The key difference for your health is that vaccines cannot replicate in your body and as such do not cause the disease that the whole pathogens do.  Most vaccines are chemical soups of disabled pathogens, containing dead bacteria, weakened viruses, or just chunks of them.  Vaccines often also include an adjuvant, which replaces the alarm signals that normally get the immune system to react to living pathogens[2].  Together, the pathogen soup and adjuvant cause the immune system to wake up and mount a response against the soup.

When the immune system begins to process the pathogen soup, that soup is chewed up by specialized cells, such as dendritic cells[3], into tiny protein bits called antigens.  Those specialized cells put the antigens into specific proteins[4] that the players of adaptive immunity, such as T-cells, can specifically see inside of.  The T-cells then wildly proliferate and send signals to the B-cells, which in turn also proliferate and begin to pump out antibody that is reactive specifically against the activating antigen and binds strongly to that antigen when it’s seen on the whole pathogen.  The bound antibody acts as a beacon for innate immune cells to home to.  Together, the T- and B-cells harness the destructive power of the innate immune system, which can chemically bomb and chew up invading pathogens and damaged surrounding tissues, to mount a precision attack on that pathogen.

The thing is, though, that vaccines don’t contain infectious pathogens, just pieces of them that the immune system processes.  When the immune system realizes that there isn’t anyone to actively fight against, it stops beefing itself up[5] and becomes quiescent again.  When it does so, some of the cells of the adaptive immune system differentiate themselves into memory cells that dwell in tissues like sentinel ninjas, silent until they see the enemy again, then they leap out and swiftly stop them in their tracks.

Vaccines skip the messiness and potential danger of an actual infection and teach the immune system how to guard against it, no lasting damage or death required.  This represents not only an incredible win for us as modern individuals, but also has inarguably made the world a better place through the elimination of smallpox, and the near-elimination of polio, whooping cough, lockjaw, scarlet fever, measles, and mumps[6].

I, for one, am happy to not have to risk destroying any of the priceless art that keeps me alive.

[1] So to speak.  The immune system has eyes in the sense that its cells have receptors that scan their environment for danger signals.  Dendritic cells do most of this.

[2] OK, so viruses aren’t technically alive to begin with.

[3] In the immunological canon, dendritic cells are the primary antigen processors, but under certain conditions other cells, including macrophages and basophils, can do the same.

[4] These proteins are called the MHC proteins, which stands for major histocompatibility proteins and act as molecular lenses for the volatile cells of the adaptive immune system (T- and B-cells) that can attack their own body if they get inappropriately activated.  That has been found to occur in conditions ranging from asthma to inflammatory bowel syndrome.

[5] The proliferation of adaptive immune system cells in an infection is responsible for the palpable swelling of the lymph nodes that we can feel under our chins or armpits when we have the flu.

[6] In the industrialized Western world, at least.