A syringe is used to expose the body to a dead pathogen and its antigens once it has reached the blood stream
once the pathogen has entered the blood stream the body makes use of the third line of defense and sends out b cells to the area of the pathogen the third line of defense is used because the first and second have been bypassed.
The B cells make antibodies to then attach to the specific pathogen after this happens the body then has a new specific defense mechanism, then memory cells are made and if the memory cells recognize the same pathogen again it will know what antibody to use .
After the body has made immunity due to being vaccinated the body then may be exposed to this active pathogen but it knows how to fight it off
The pathogen has passed through the first 2 lines of defense then the third line of defense and then sends b cells to the pathogen that recognize the pathogen and can then attach to the pathogen
Once the Antibodies have locked onto the pathogen the other immune cells and the non specific defense mechanism help to kill the pathogen,