LCK and rheumatoid arthritis: We demonstrated that IgD-Fc-Ig (composed of human IgD Fc domain and IgG1 Fc domain, obtained through prokaryotic protein expression and chromatography purification) effectively inhibited the activation and proliferation of T cells in healthy controls and PBMCs in RA patients stimulated by IgD, recovered the Th17/Treg cell subset balance, and downregulated p-Lck and p-ZAP70 expression.