I have read that a vaccine against a pathogen typically works by using a dead or weakened version of that pathogen and then inciting an immune response against the pathogen so that the immune system then recognizes that pathogen in the future and attacks it immediately.

However, suppose that a person gets vaccinated against a virus. Why couldn't the virus infect a few cells before it runs into immune system cells and starts replicating as normal? Does the immune system somehow recognize virus-infected cells?

Also, how would a vaccine against a virus that attacks immune system cells work (like HIV, for instance)? In this scenario, wouldn't any immune cell that tries to destroy the virus get infected itself?

  • $\begingroup$ Look at the normal reaction of your body during infection by a new virus, how (the steps are : detection of the infection, inflammation, processing and presentation of antigens, replication, mutation and selection of the B-cell-antibody pool) it needs 2 weeks to create potent B-cells-antibodies, how it is much faster when enough of the viral proteins contain "antigens" already seen for which your body previously made long-term memory B-cells. $\endgroup$ – reuns Apr 6 at 0:55

Vaccines containing these weakened or killed viruses or bacteria are introduced into your body, usually by injection. Your immune system reacts to the vaccine in a similar way that it would if it were being invaded by the pathogen — by making antibodies. The antibodies react to the vaccine (virus/bacteria) just as they would the live pathogen — like a training exercise against an antigen. Then they stay as amemory in your body, giving you immunity. If you are ever exposed to the real disease, the memory is activated and antibodies are there to protect you.

Moreover if a cell gets infected virally, infected cells produce and release small proteins called interferons, which play a role in immune protection against viruses. Interferons prevent replication of viruses, by directly interfering with their ability to replicate within an infected cell.

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  • $\begingroup$ It is better to use the term "pathogens" instead of "germs" especially when the OP is very well aware of the former. Also, it would be great if you could provide some references $\endgroup$ – WYSIWYG Mar 10 '19 at 18:46
  • $\begingroup$ Edited your answer for terminology. If not acceptable, please feel free to roll back. $\endgroup$ – anongoodnurse Mar 11 '19 at 5:49

It depends on the particular vaccine. Generally we can divide vaccines into two groups: live and "dead".

Live vaccine contains weakened (attenuated) pathogen, which (in case of a virus) theoretically can infect our cells. However, as it is attenuated, it is not doing it very efficiently, thus even naive immune system has no problem with defeating it.

"Dead" vaccines contain either inactivated ("killed") pathogen or only some of its parts (antigens). As it is not active virus, it can not infect your cells. It is just presented to your white blood cells as a "dummy" for "target practice". And by "target practice" I mean things like activation and proliferation of competent lymphocytes, followed by formation of immunological memory - exactly as if the organisms was infected by a virus.

Our body is "blindly" creating a variety of lymphocytes, and each of them is able to fight with pathogens with certain antigens. As the lymphocytes has been created "blindly", most of them seems to be useless. However, if there happens to be a lymphocyte able to recognize a virus antigen, the lymphocyte is indeed useful. It proliferates, "learns" to recognize the antigen even better, fights the virus (or its antigens) and is preserved as memory cells, just in case of reinfection.

Of course I am oversimplifying here, but I hope I managed to grasp the big picture. ;)

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