The enzyme Der p1 is made in the gut of the house dust mite and then excreted into its dropping. The droppings are small enough to be breathed in and dissolve in moist conditions. In these conditions, and in a concentrated form, Der p1 rapidly escapes for dispersal. In the environment the majority of the fragmented enzyme will attach to large particles. However, Der p1 has been found on tiny fragments of fibers and various flakes, all of which can be inhaled more readily.
The reason why the mite has such a powerful enzyme is based in its evolutionary history. About 86 million years ago the mite changed from being a mange-like parasite (feeding on skin sores) to become a scavenger of discarded skin scales and organic debris in the nests of warm-blooded animals. In making the transition the mite kept its powerful digestive enzyme to break down hard-to-digest food. The mite considers its droppings as a future and nourishing food. In fact, the enzyme has been found to be similar to a meat tenderiser (papain) used in the food industry. Unfortunately for humans, the active enzyme is known to cause Itching, sneezing and wheezing, either as an allergen, an irritant, or both.
Intact house dust mite dropping encasing the mite’s active digestive enzyme Image by Dr Harry Morrow Brown and gifted to housedustmite.com
Scientists who study the enzyme’s activity as a cause of allergy consider it an ‘initiator allergen’. In other words, once Der p1 has caused cell damage any bystander can take advantage of the sudden breach in the body’s defences. The new invader may cause the immune system to react by becoming suspicious or sensitised, which is the first step in becoming an allergen. Please note, it is by this route that the mite’s second major allergen Der p2 (mimicking bacteria) is detected.
Reference Int J Mol Sci. 2018 Nov; 19(11): 3549
It is worth noting here that Der p1 has been identified in the human gut, and in breast milk. It is very invasive enzyme indeed!
Hardcore information. Der p1 is a glycoprotein with sequence homology and thiol protease function similar to the enzymes papain, actinidin, bromelain and cathepsins B and H.
Allergens and Allergen Immunotherapy; Fourth Edition, 2008, Chapter 10, Mite Allergens, Enrique Fernandez-Caldas, Leonardo Puerta, Luis Caraballo, Richard F Lockey, Publisher Taylor & Francis. 'Clin. Allergy Immunol. 2008; 21: 161-182
'Detection of House Dust Mite allergen in amniotic fluid and umbilical-cord blood'; Holloway J A et al, The Lancet, 2000, Issue 9245, 1900-1902.