Researchers have found a potential link between allergies and asthma, and rheumatoid arthritis (RA) after adjusting for factors such as urban environment and exposure to secondhand cigarette smoke, recent research has shown.

While other studies have linked asthma and RA, one major limitation of these studies is they did not adjust for allergic disease, secondhand smoke exposure or pollution in urban areas, which are “known contributors” to the development of RA and asthma, Vanessa L. Kronzer, MD MSCI, from the Mayo Clinic Division of Rheumatology, in Rochester, Minnesota, and colleagues said.

Man with allergic asthma. Souce: Getty

“The association between asthma and RA is potentially biologically plausible. Both are immunological disorders profoundly influenced by environmental factors that induce oxidative stress such as cigarette smoke, wood smoke, and environmental pollution,” Dr. Kronzer and colleagues wrote in their study, which was published in the journal Arthritis & Rheumatology. “Moreover, there are many potential mechanisms that may mediate both disorders, including Th17 inflammation, infectious triggers, premature immune senescence, or other inflammatory mediators, such as TNF-alpha and leukotrienes.”

The researchers performed a case-control study of questionnaire data from 1,023 RA cases (175 incident cases) from the Mayo Clinic Biobank repository between April 2009 and December 2015. Patients were at least 18 years old and self-reported their RA disease status, with the researchers performing a manual chart verification of 100 patients to ensure the accuracy of RA status in the cohort. The questionnaire asked patients about their history of asthma, history of allergy, smoking status, and passive smoking exposure.

They found an association between RA and asthma in the full cohort after adjusting for factors such as allergies, urban pollution and passive smoke exposure (odd ratio, 1.28; 95% confidence interval, 1.04-1.58). However, this association was not present in the incident cohort of RA patients (OR, 1.17; 95% CI, 0.66-2.06). Allergic disease was also associated with RA in the full cohort (OR 1.30; 95% CI, 1.12-1.51) as well as the incident group (OR, 1.61; 95% CI, 1.11-2.33). In particular, food allergy was associated with RA in the full cohort (OR, 1.38; 95% CI, 1.08-1.75) and the incident cohort (OR, 1.83; 95% CI, 0.97-3.45).

“[T]he presence of a positive association between allergy and RA was initially unexpected since current immunologic theory segregates RA into the Th1 pathway and allergy into the Th2 pathway,” Dr. Kronzer and colleagues wrote.

Age at beginning smoking or exposure to secondhand smoke in a home or work environment were not associated with RA in either the full cohort or incident RA groups. The researchers noted future studies are needed to further study the relationship between early life atopy and RA.