Your body has only a few emergency levers it can pull. It also only has a few detection systems.

It takes time for the immune system to respond. It has to recognize a problem, communicate to the immune system generals, mobilize the early troops and then start producing the rest of the defense warriors.

Mucus production is really quick to start. The detectors are nearby and all that needs to be done is open up a tap and the mucus starts flowing. It provides a protective barrier between the environment and body tissue, plus it also has a flushing effect as it moves out and down.

Nasal congestion not only is just for viruses, it is also for bacteria, pollen and foreign objects. Sometimes a good flushing is all that is required.