Mites
According to several studies, HDM sensitization and allergy are considered to be the most frequent among inhalant allergies [
5,
17,
18]. In our conditions, it holds the fourth position behind grass pollen (sensitization frequency for Phl p 1 is 60.8%), birch pollen (sensitization frequency for Bet v 1 is 47.3%), and cat allergens (sensitization frequency for Fel d 1 is 31.5%) [
6], what differs from data coming from other regions [
17,
19,
20]. This discrepancy may be caused by geographical differences or may be due to the selected population, mainly consisting of adults with predominant respiratory allergy. Higher sensitization rate to grass pollen in adolescents was also shown in a study from northern Italy while the leading position of mites was pronounced clearly in smaller children [
19].
It is known that patients sensitized to mites are not always sensitized to the molecules used in our study, other molecules may also play a role, but the number of patients sensitized only to these other molecules is generally very low [
21‐
25]. Moreover, cysteine protease (Der p 1, Der f 1) signaling has been described to have a strong TH2 up-regulation effect, and group 2 allergens have been shown to bind TLR4 via binding LPS, thus having a stronger immunogenic potential further enhancing the complexity of mite allergy. Hence, these molecules may play a leading role in the atopic march, and a differential sensitization rate in children compared to adults may be present [
26]. Sensitization to group 1 allergens was shown to be more frequent in children [
18], thereby suggesting a possible role of group 1 allergens in the onset of sensitization, perhaps mediated by their proteolytic activity and direct epithelial damage [
27]. The more frequent sensitization to group 2 allergens in the adult population suggested a later sensitization to group 1 allergens what was documented by observation of a birth cohort [
26]. The ability of group 2 allergens to bind directly to TLR4 might be an explanation for this observation. These facts may also explain that in the case of mites the higher the exposure, the more severe the clinical allergic condition, in contrast to cat allergens where overexposure seems to lead to tolerance [
28].
Dermatophagoides pteronyssinus is the most widespread mite all over the world; it predominates especially in humid regions where the climate is more influenced by the ocean.
Dermatophagoides farinae is supposed to occur more in the continental regions of Europe and the Mediterranean area, but most countries have mixed populations [
22,
29]. In our patients, sensitization to
D. pteronyssinus and
D. farinae was almost equal in contrast to Spain where the IgE-prevalence to
D. pteronyssinus allergens was found to be slightly higher than to
D. farinae allergens [
18].
Although the pyroglyphid HDM
D. pteronyssinus and
D. farinae seem to predominate, glyciphagid mites may also be important in some regions [
18,
21,
29].
Blomia tropicalis is the most important house dust mite from the family glycyphagidae. It is most abundant in tropical regions [
4,
29]. The major allergen Blo t 5 shows 40% sequence homology with Der p 5, but it was reported not to cross-react with each other [
29]. As
B. tropicalis does not form part of the acarofauna in Central Europe, the not negligible sensitization rate in our patients seems to be attributable rather to potential cross-reactivity among allergens produced by different mite species (mite group 5/21 molecules) than to true sensitization. This assumption is also supported by low levels of sensitization to Blo t 5 in our group. The results of another study suggested that allergens different from those belonging to group 5 may also be responsible for the partial cross-reactivity among different mite species [
30].
Lepidoglyphus destructor, a glycyphagid storage mite, may also become a HDM [
29]. Sensitization from domestic exposure was reported from Sweden and France. It is not possible to decide whether positivity to Lep d 2 in our patients was due to true sensitization or cross-reactivity within mite group 2 molecules. The absence of mono-sensitizations to Lep d 2 (in the frame of mite-derived molecules) in our patients testifies rather for Lep d 2 sensitization due to cross-reactivity with other group 2 allergens, what is in contradiction to the data suggesting that no such cross-reactivity exists [
18].
Mite sensitized patients in our group were usually co-sensitized to several mite-specific components; monosensitization was markedly less frequent (Fig.
2). Der f 1 and Der p 1 (cysteine proteases) and Der f 2 and Der p 2 (lipid binding proteins) are assumed to be the specific components most commonly involved in mite allergy. The predominant sensitization, both regarding prevalence and intensity, to mite group 2 molecules was already described [
17,
18,
25] and confirmed by our results. In a French study, predominant sensitization to Der p 1 (93%) was detected [
31], and we may speculate that this difference might be due to the different population studied. The same French study also detected a higher frequency of sensitization to Der p 10 (28%) signaling the primary sensitizing agent possibly being shrimp which is much more consumed in the Mediterranean area than in Central Europe.
Tropomyosins are molecules responsible for cross-reactivity among mites, shrimp, and cockroach [
32]. IgE binding to the mite group 10 allergens is rare in Europe [
21] and Australia [
33] and, from one study, in US subjects allergic to both HDM and cockroach [
34]. Similarly, in this study’s population group, sensitization to individual tropomyosins was considerably less frequent than sensitization to more tropomyosin molecules together suggesting present cross-reactivity in the frame of this group of molecules (Fig.
3).
Sensitization to the tropomyosin Der p 10 was not observed frequently (in 1.9%) in our patients, and co-sensitizations with other mite-specific molecules were not regular (in 0.8%) suggesting the possible different route of sensitization in a considerable proportion of the patients. Tropomyosins represent clinically relevant seafood allergens, but the role of mite tropomyosin, Der p 10, in house dust mite allergy has not been studied in detail. A hypothesis that tropomyosin sensitization may indicate a true food allergy independent of mite respiratory disease has been proposed [
18]. Another hypothesis pretends that Der p 10 may be a diagnostic marker for mite-allergic patients with additional sensitization to allergens other than Der p 1 and Der p 2. Such patients may require attention when allergen-specific immunotherapy is considered [
35].
The high rate of simultaneous sensitizations to different mite components (Fig.
2) may be explained either by co-sensitizations or by cross-reactivities. Specific IgE to Der p 2/Der f 2 is almost fully cross-reactive, but no cross-reactivity was described with Lep d 2 [
18]. Group 1 allergens are also cross-reactive, but in some patients, a species-specific response was observed [
18]. Relatively high rate of sensitization both to group 1 and 2 allergens in our patients indicates the greater role of co-sensitizations.