First of all, a great deal of praise for Luise Schulz’s reply.She has already explained very nicely what the difference between passive and active vaccination is and how they work. This answer here can only be seen as a supplement, perhaps a little more technical, to active vaccinations.
But first of all the course of a natural infection:
If a person is infected with a pathogen, say a virus, it first penetrates into a cell.There, it causes the cell to multiply by reprogramming the cell. The newly created viruses leave the cell, for example, in which they cause it to burst, and set out to infect new cells. Without the immune system (IS), this would continue to be the case.
IsIS, however, is not standing idly by. If a cell detects that it is infected, it signals it to ISIS. This then grabs parts of the virus and begins to produce antibodies.This is done as follows: antibodies are randomly produced and then tested to see if they detect the snapped piece of the virus. Those who do not are destroyed again, those who detect the virus are multiplied and improved, so that in the end an antibody is found that binds well to the virus. This antibody is then produced millions of times by special cells and the virus is shot down. Once the right antibody is found, the elimination of the virus proceeds very quickly. But until this antibody is found, several days pass during which the virus reproduces and during this time often breaks many body cells (these damages are not always repairable). If the immune system allows it to find the right antibody for too long, it loses and the person dies.
However, when everything works out and all viruses are eliminated, IS IS remembers which antibody was particularly successful against the virus and stores the blueprints for it in special cells. The longer the struggle lasted, the more such memory cells are formed.
If there is now a new infection with this virus years later, these memory cells are brought out and produce millions of the right antibodies again in a hurry.The cumbersome and lengthy search for the right antibody is thus eliminated and a lot of time is saved in which the virus could multiply and cause damage.
So much for the natural development of immunity in successful defense of a pathogen.But now to the vaccination…
Vaccination basically does nothing more than simulate this first infection, but without the risk that the pathogen can only multiply unchecked.The pathogen is shown to ISIS, but it can only multiply poorly (in live vaccines) or not at all (in dead vaccines). Sometimes ISIS is even presented with only parts (say, the shell) of the virus (subunit vaccine). Each of these methods has its advantages and disadvantages, which must be weighed against each other in the development of vaccines on a case-by-case basis. The choice of method then also decides on the addition of excipients that must be used.
If you come into contact with the right pathogen after vaccination, the right antibody is already available and the procedure corresponds to the second infection.This saves valuable time and prevents damage from the pathogen.