Background: According to the T-helper (Th)1/Th2 paradigm, it has been argued that Th1-dominated diseases such as psoriasis and Th2-dominated diseases such as asthma and hay fever should be mutually exclusive, as the immune regulatory cells, Th1 and Th2, cross-regulate each other. An increase in asthma and hay fever has occurred in Sweden over the past four to five decades, but the time trend for psoriasis is not clear.
Objectives: To assess the prevalence of psoriasis in young Swedish men over a period of three decades and the association between psoriasis and allergic disorders.
Methods: Register study based on data from the Swedish Military Service Conscription Register and the Total Population and the Population and Housing Censuses. Psoriasis, asthma (with and without allergic rhinitis) and allergic rhinitis at conscription were studied in 1 226 193 male conscripts in successive cohorts born between 1952 and 1977.
Results: The prevalence of psoriasis was about 0.5% and it remained stable over three decades. Conscripts with psoriasis and their siblings were less likely to have eczema. The reduced risk of eczema in conscripts with psoriasis was unchanged over time. A reduced risk of allergic rhinitis was also demonstrated in conscripts with psoriasis and their siblings but only in the most recent birth cohort born between 1970 and 1977.
Conclusions: A strong genetic influence may explain the unchanged prevalence of psoriasis in Swedish conscripts. A genetic predisposition for psoriasis may confer partial protection from eczema and allergic rhinitis. The inverse relationship between allergic rhinitis and psoriasis appeared to be a recent phenomenon.