Before this life-style shift the mite survived by placing digestive enzymes in its droppings to irritate the skin of the host causing itching and weeping sores which then fed the mite. A whole HDM colony could live under crusted wounds made by the enzymes until the animal died, signalling death to the mite and its colony. Not a very good prospect, so a survival shift was made by this clever mite from mange-like parasite to scavenger. An example of this type of mange mite living today is the HDM’s close relative, the sheep scab mite. This well-known pest causes acute allergic dermatitis in goats and sheep and is a serious veterinary problem for farmers worldwide.[3,4]
The house dust mite was now living in the nesting sites of birds or rats scavenging on whatever food was available, such as discarded skin scales, fungi, bacteria, bits of food, dead insects or pollen.  In order to digest such a wide-ranging diet the mite relied upon several specialized gut-derived enzymes to do the job, many are placed in its droppings to break down left-over food for later nourishment. Like rabbits, HDM consider their droppings as food. Travelling from nest to nest was easy for the mite because when the animals moved, they took their mites with them to new warm damp, nesting conditions. For the host animal the disadvantage was that the unwelcomed mite, its colony and droppings, caused irritation and/or sores.
HDM Dermatophagoides pteronyssinus - Courtesy of the LSHTM
Recently one of the mite’s digestive enzymes (which has biochemical properties) was investigated in detail and found to reflect the mange-like past of the house dust mite.[7,8] The enzyme named Der p1, is made within the mite’s gut and is similar to one made by the sheep mange mite. Both enzymes (sheep and mite) are compared to papain, a meat tenderiser used in the food industry.[4,9]
Although Der p1 is a single enzyme it has many tasks. It is first out of the mite’s dropping as it dissolves in moisture. Its first task is to release the activity of companion enzymes or products held in the dropping. The second task of Der p1 is to go to work breaking down the protective surface of defence cells or the ‘glue’ that binds delicate cells together. This activity offers bystanders (other mite products, dirt, or bacteria) an entry into the body.  This activity is selective and can raise immune signals that are associated with allergy, or an allergic reaction to the mite.
Activated enzymes/allergens can deepen a reaction by attracting non-allergic defence mechanisms that can harm nearby healthy cells through a cascade of unwanted reactions.  These non-allergic responses are associated with lung diseases such as non-allergic asthma and COPD.  Fortunately, nature has balances that can dampen down this harm. Der p1, however, can interfere with this dampening down process, thus allowing its full damaging force.[13,14,15]  This was Der p1’s third task, to keep a harmful immune reaction rolling on.
Reviewing three tasks of Der p1
Quote: “From the viewpoint of the organisms that actually produce allergens, these proteins exist only to function as enzymes and structural or regulatory proteins. The exception is the allergen of parasites that may modulate, and thus subvert, host protective immune effector mechanisms.”
Reviewing the activity of enzymes produced by HDM and the statement above, a question should be asked. Is the house dust mite a scavenger with harmful parasite-like functions, and is this why it is a cause of disease worldwide?