Common immune-related exposures/conditions and risk of non-Hodgkin lymphoma: a case-control study of disease-discordant twin pairs.

We evaluated the association between common immune system-altering experiences and non-Hodgkin lymphoma (NHL) risk using a case-control study of 162 like-sex twin pairs discordant for NHL, identified from the International Twin Study. Information on medical history and evidence of childhood exposure to microbes was obtained by questionnaire from 1998 to 2002. Conditional logistic regression was used to estimate odds ratios and 95% confidence intervals. Intra-twin-pair agreement between twins on individual exposures was high (76%-97%). A negative association between NHL and seasonal hay fever (odds ratio (OR) = 0.28, 95% confidence interval (CI): 0.10, 0.75) and certain allergies (OR = 0.29, 95% CI: 0.13, 0.68) was observed. The number of atopic diseases was negatively associated with NHL (P for trend = 0.0003). A history of infectious mononucleosis was negatively associated with NHL risk (OR = 0.35, 95% CI: 0.14, 0.90). NHL risk was associated with more frequent childhood exposure to microbes during early life (P for trend = 0.04). No differences in association by NHL subtype were observed, although statistical power for these comparisons was low. These observations support the hypothesis that immune-related exposures, especially atopy, are associated with decreased NHL risk. Use of the within-twin-pair study design mitigates confounding by genome, family structure, and unmeasured characteristics of early childhood factors.