22
Dec
2018
F
Cr
Feature Review
Clinical
In the early phase reaction, processed
allergen peptides presented by dendritic
cells to ‘primed’ mast cells causes release
of preformed mediators: vasoactive amines
(histamine), enzymes and proteoglycans.
Histamine is the mediator of most
relevance in the development of symptoms
and in the treatment, causing smooth muscle
contraction, increasing vascular permeability
and promoting mucous secretion.
The late phase response is induced by
synthesised chemokines, some of which amplify
the early phase response, and others that
attract and promote migration of leukocytes
(eosinophils in particular) without further
antigen exposure. 7
Genetics
This is a multifactorial disease with a clear
genetic role in its aetiology. Allergic rhinitis,
asthma and other atopic diseases show strong
familial and intra-individual clustering. 8
Numerous loci of the HLA gene within
chromosome 6 have been implicated in the
pathogenesis of atopic diseases, particularly
in the propensity toward TH2 compared with
TH1 immune responses.
Allergens
Inhaled aeroallergens are the principal
environmental factor in the pathogenesis of
allergic rhinitis.
Allergens are classically divided into indoor
(house dust mites, animal dander, insects and
plant matter), outdoor (pollens and moulds)
and occupational (chemicals, bakery allergens,
laboratory animals).
Outdoor allergens typically result in seasonal
variation of symptoms while indoor allergens
are more likely to cause persistent rhinitis.
However, up to 50% of patients sensitive to
pollens have persistent rhinitis, most likely due
to multiple pollen sensitivities or sensitivities
to other indoor allergens. Likewise, a large
number of patients sensitive to house dust mites
experience a mild intermittent allergic rhinitis. 8
Inhaled aeroallergens have a significant
geographic and seasonal variation so it is
important for practitioners to understand their
region’s prominent allergens.
Figure 1. IgE-mediated type I hypersensitivity reaction in allergic rhinitis.
House dust mites
The most important species, Dermatophagoides
pteronyssinus, D. farinae and Euroglyphus
maynei feed on human skin dander, plentiful in
mattresses, sheets, pillows, toys and furniture.
They prefer humid (higher than 50%), warm
(above 20°C) environments. House dust mite
allergens are found in mite faeces, made airborne
when contaminated fabrics are disturbed. These
allergens quickly settle. Higher concentrations
of mites are associated with an increased risk of
developing asthma at a later date. 8