Immune deficiency and altered immunity are among the best characterized and strongest known risk factors of non-Hodgkin lymphomas (NHL). For instance, chronic inflammation or certain disturbances in the immune system are associated with an increased lymphoma risk. Occupational and environmental factors (i.e., dioxin) as well as lifestyle factors (i.e., obesity) may contribute to these risk factors. The precise role of these factors in the etiology of NHL, however, is still not entirely clear. Although the existing epidemiologic studies have not revealed consistent patterns of perturbations of the immune system by these factors, the findings might suggest an adverse impact on both the humoral and cell-mediated immune system.