Allergic reactions have evolved to survey and protect the host from the environment including the air, water and food. They are unique in the fact that they only happen in the defence barriers to the outside world and are designed not only to rid perceived harm but to trigger avoidance tactics so that the uncomfortable experience is not be repeated. Allergens that cause the reactions are all non-infectious environmental stimuli, therefore seemingly harmless. Why allergens become a problem remains to be defined, however excessive exposure to allergens can lead to overlapping of immune defences that result in allergic disease.
How do house dust mites fit into this scenario? To understand the full impact this mite can have on health we need to go back to the beginning. To simplify things there are two separate defence mechanisms. Type 1 immunity that acts against microorganisms such as viruses, bacteria, fungi and protozoa while Type 2 immunity acts against macroparasites such as helminths (worms) and ectoparasites such as lice and ticks. It is important to note that Type 2 can also act against a broad range of environmental challenges that if exacerbated can cause overlapping and chronic allergy. These two immunities have separate arsenals of weapons available to restrict, kill or repel invaders, but when acting together can be problematic.
Allergens from house dust mites can cause both Type 1 and Type 2 immune reactions in highly susceptible patients.
What are the authors of this article saying? Listen to your unique immune system, don't silence initial allergic reactions but heed their warning, find the cause and try to make environmental changes. Allergic reactions can be beneficial.
The article referenced is highly recommended for those interested in studying allergy.
'Allergic host defences', Noah W Palm, Rachel K Rosenstein, Rusian Medzhitov, 'Nature' | Perspectives, Vol. 484, 465-472 (April 2012)