High serum IgG4 concentrations are a striking feature of many patients with IgG4-related disease (IgG4-RD). Blood levels of IgG4 often reach ten, twenty, and even thirty or more times higher than the upper limit of normal. Under the proper clinical circumstances, the finding of an elevated serum IgG4 concentration serves as a useful biomarker for the diagnosis of this condition. This serum IgG4 elevation quickly called attention to the possibility of therapies targeting cells of the B lymphocyte lineage. In addition, a greater understanding of the cellular mechanisms that underpin IgG4-RD has identified peripheral blood plasmablasts as a promising biomarker for this disease. The roles of plasmablasts and B cells in IgG4-RD are discussed in this chapter.