Thursday, July 9, 2015
Respiratory allergy caused by house dust mites: What do we really know?
The house dust mite (HDM) is present in human habitats around the globe, and it is a significant factor in allergic rhinitis and allergic asthma. Sensitization to mite allergens in early life compromises lung function and leads to wheezing in children, and it associates with poorer outcomes in a patient’s respiratory health in the long term. Calderón et al review the epidemiology of HDM allergy and the effect of HDM allergens on the human immune system (J Allergy Clin Immunol 2015; 136(1): 38-48).
Both assessing prevalence of HDM sensitivity and controlling a patient’s exposure to the allergen pose challenges. Prevalence data for HDM sensitization vary according to targeted population: from estimating 65 million to 130 million people in the global general population may be affected to as many as 50% of those with asthma. In addition, results vary within geographic locations, meaning studies have found significant differences in prevalence within a given country or region. In terms of a patient’s exposure, house dust mites are ubiquitous. Humidity levels have been shown to affect HDM propagation, and a recent practice parameter recommends the use of a hygrometer in the home. Studies indicate that HDM allergen levels should be maintained at less than 2 μg/g to decrease the likelihood of sensitization, yet measures to decrease HDM exposure have shown little benefit on symptoms in sensitized patients. Finally, the quantitative relationship between exposure to HDM and symptoms in asthmatic patients is complicated, as many of these patients are sensitized to more than one allergen.
Allergenic effects in HDM allergy are thought to be orchestrated via two main routes: through the CD4+ TH2 cells that induce and drive the IgE-dependent allergic response and through the innate immune system. It is this combined effect of adaptive and innate immune reactions that makes the allergen so powerful. Current guidelines for allergic rhinitis and allergic asthma classify disease based on severity of symptoms. That HDM is often the underlying cause is an important step in managing clinical control, as well as potentially preventing disease progression.