immune system response and reinfection

[the guy]

When the body has developed an immunity to a particular strain of virus, either through past infection or vaccination, and the virus enters the body once again it is obviously overcome before it can have any harmful effects.

 

What I was wondering is whether such viruses manage to infect any cells before the immune response takes effect, or if they are totally overcome before this can happen?

[Ringer]

The virus can infect cells before there is a proper immune response, that's how many people get sick when stressed, tired, etc. The immune response isn't working as effectively and infection takes place before a proper immune response begins. Viruses also have a high mutation rate, so the virus you may be immune to may be vastly different than the same virus you get infected with a year later.

[CharonY]

Also note that the immune response is (as all biochemical reactions) a bit of a stochastic effect. I.e. even if a given antigen has been memorized by the immune system, it does not mean that the response is immediate and 100% effective. So at very low concentrations the immune system is likely not to mount any response at all (but then the likelihood of succesful infection is also not very high). But once infection is successful and the virus particles start to spread (or the titer was high to begin with), the vaccinated immune system will have a much higher chance to contain the infection.

It will depend on the precise mechansims of the infection process, however.