The allegory of the lookout and post-exposure immunization

This allegory is supposed to bring light upon the phenomenon of post-exposure prophylaxis. How can an insertion of an inactivated or attenuated disease virus into the body ultimately be helpful, even though that disease already is present in the body? It is simply said that the vaccine triggers an immune response more rapidly than the natural infection itself. However, the truth is more complicated than that. In fact, it is yet not fully understood. Therefore, this allegory isn't fully explaining the true mechanism, but at least tries to explain the principle.

The lookout
Imagine there is a fortress, named the body. This fortress is regularily attacked by different types of invaders, named diseases. To cope with this problem, the fortress has evolved a defense system, named the immune system. However, it is a hard work to keep defense system up and running, so to be able to support all other functions of the fortress, the defense system is inactive most of the time. Instead, a lookout is placed on the fortress walls. When an invader breaks in, the lookout sounds the alarm and the defense system takes care of the invader. The name of the lookout, however, is still unknown.

The invader
Anyway, some invaders, e.g. rabies virus, have a method of bypassing the lookout unseen - a method so genious, that still today it is unknown. The alarm is never sounded, and the immune system remains inactive.

The post-exposure immunization
Physicians, on the other hand, can suspect that an invader has entered the fortress. They take a couple of invaders from elsewhere, and subsequently dump them into the fortress by a syringe so large that it goes all the way through the fortress walls. This is impossible for the lookout to miss, so the alarm is sounded, and the defense system takes care of not only the injected invaders, but also the invaders already inside. In other words, the genious method of the invaders to get inside unseen is, in turn, bypassed.

Reality
However, there are many aspects needing further research. What is the nature, for example, the molecular structure, of this "lookout". And what stage of invasion is this "bypassing of the fortress wall unseen", which for example rabies viruses accomplish when entering by a bite of a dog, but fail when injected? If this could be explained, more effective methods of post-exposure immunization can be applicated.