Malassezia spp.
Summary
Fungal species belonging to the genus Malassezia are part of the normal microbome of healthy human skin. More than a dozen species are identified, the species most commonly found on healthy human skin are M. sympodialis, M. globose and M. restricta. There is evidence of an association of hypersensitivity to Malassezia-specific allergen and atopic dermatitis. A similar association with allergic respiratory disease is also proposed. Over three dozen Malassezia specific allergens are recognized belonging to M. furfur, M. sympodialis and M. globose species. Most known cross-reactivity is seen with allergens of other fungal species.
Epidemiology
Worldwide distribution
There are over a dozen species of Malassezia, the majority of which are isolated from human skin. M. globosa is the dominant species worldwide (except for Canada and Indonesia), a degree of geographical variation of the prevalence of Malassezia species exists. In the early 1990s, prevalence of M. sympodialis in healthy human skin cultures was between 7–69%, with the highest prevalence found in Sweden and Canada. Whereas an Iranian study at the turn of the 20th century found M. furfur with a prevalence of 4–23%.
A high frequency of sensitization to Malassezia allergens in patients with head and neck type atopic dermatitis (55–68%) has been reported. However, it has also been suggested that the Malassezia population on lesional atopic dermatitis skin is reduced compared to healthy skin.
Route Of Exposure
Main
The mechanism by which fungal infections of the skin can provoke hypersensitivity disease of the skin, or the respiratory tract is not clear. It is possible that the direct contact of allergens on the skin could trigger a Th2 cascade response and the link between atopic dermatitis and bronchial asthma, if proven, driven by cytokines derived from epithelial cells such as TSLP, Il-33 and IL-25.
Clinical Relevance
A strong association of sensitization to Malassezia spp. allergens in patients with atopic dermatitis has been demonstrated in several studies. Casagrande and colleagues showed that circulating allergen specific IgE was associated with patients with atopic eczema. They reported that 52/97 patients with AE had detectable M. sympodialis-specific IgE, compared with 4/571 patients with other allergic diseases and 0/38 healthy controls.
Sweat may increase the growth rate of M. globosa, as the main allergen of M. globose, MGL_1304, has been identified in human sweat. This could explain how cholinergic urticaria, with a prevalence of up to 20% in young adult, is triggered by sweating, heat, and exercise.
Cross-Reactivity
Malassezia specific allergens contain proteins with high sequence similarity to proteins from non-Malassezia genera. This is consistent with the general observation that the majority of fungal allergens have extensive cross-reactivity with fungi that are taxonomically unrelated. Consequently, Mala f 2 and 3 (peroxisomal membrane protein, or PMP), Mala s 6 (cyclophiline), Mala s 10 (Heat Shock Protein 70), Mala s 11 (MnSOD) and Mala s 13 (thioredoxin) all have the potential for cross-reactivity. The latter also has similarity with human proteins.
References
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