In this study, IgD significantly enhanced the proliferation of CD4+ T cells and increased levels of inflammatory cytokines including IL-α, IL-1β, IL-6, IL-8, and TNF-α secreted by CD4+ T cells, which was consistent with previous reports on PBMCs from RA patients (Chen et al., 2009; Wu et al., 2016). Here, IL6 is linked to rheumatoid arthritis.