Why do we have allergies?

How Do Common Allergies Develop?
Under healthy conditions, humans and bacteria living with humans have a peace agreement. Specifically, the good bacteria in the airway produce chemical signals to tell the immune system that they are not harmful. As a result, our immune system calms down and will not fight the beneficial bacteria. Because it is in a quiet mode, the immune system will not attack other things that are not dangerous, such as pollen.

Now the bacteria responsible for calming down the immune system is vulnerable to a lot of things. For example, antibiotics can kill or suppress the good bacteria to a low level. When suppressed, these good bacteria will not be able to produce enough peaceful signals to the immune system. The defense system becomes hypersensitive without sufficient signals from the good bacteria. Given enough time, the sensitive immune system will gradually produce successful antibodies against allergens, such as pollens, pet dander, and dust mites, to name a few.

The immune system starts battles or wars depending on the characters of the allergen and the quality of the antibodies against them. The fighting causes local or systematic inflammations. Consequently, the human host will experience mild or severe allergy symptoms.