We are not allergic to the mites themselves, which are up to 2 tenths of a millimetre (200 microns) long, but to allergens they secrete. This is why is is possible for a bedding cover to be genuinely “mite proof”, but have no clinical benefit. Over 20 different mite proteins have now been identified as “allergens”, which means that allergic people can produce allergy-type antibodies to them (immunoglodulin E – IgE). Each allergen is described by the first three letters of the mite’s scientific name Der for Dermatophagoides (the genus), the letter p for pteronyssinus (the species most common in the UK and Europe) and a number. The number has nothing to do with that particular allergen’s clinical importance, but is purely assigned according to the order in which it was discovered.
Different people have differing amounts of antibody to different allergens. Many have none at all to some of the allergens. All clinically sensitive patients have antibody for
Der p 2, which is probably the most important single allergen.
Tests for mite allergens in dusts usually only measure the faecal allergen “Der p 1”. This is because it was the first mite allergen discovered and ways of measuring it were produced. All the initial scientific epidemiology concerning mite exposure and asthma/allergy risk used this technique. It is still a useful measure of natural mite allergen exposure as there is a very strong correlation between the amount of Der p 1 in house dusts and the amount of other allergens, but this does not mean that showing something filters out Der p 1 means it will protect you.
Many people are aware that some of the mite’s allergens come from their droppings (faecal particles). The overwhelming proportion of Der p 1 in dust is in intact faecal particles, which are too large at over 40 microns to be breathed into the lung. Der p 2 is not a faecal protein and is carried on smaller particles. Particles capable of entering and being deposited in the lung are “respirable” particles, between 0.5 and 20 microns size. It is therefore possible to show that a product removes, or filters out, 99% of Der p 1, while it still lets through particles of less than 20 microns to cause asthma.
Particles smaller than 0.5 microns can be breathed in, but because of their aerodynamic properties, do not deposit in the lung and are breathed out again. This is why it is important that respiratory protection products cover the whole range of particle sizes between 0.6 and 20 microns. Advanced Allergy Technologies’ products are tested with Der p 1 and Fel d 1 (the major cat allergen which is on particles which go down to 0.2 microns), and with Sodium Chloride particles of 0.6 microns.