PTPRC and melanoma: While there were no significant differences in the numbers of circulating B cells (CD45+CD22+CD19+CD3−CD14−) in the blood of patients with melanoma compared with healthy volunteers (Fig. 3A), the proportion of IgG4+ cells in the peripheral blood B cell compartment (CD45+CD22+CD19+CD3− CD14−) of melanoma patients was significantly elevated (n = 47; median 0.50; 95%[CI] 0.47–0.73) compared with that of healthy volunteers' (n = 24; median 0.2; 95%[CI] 0.13–0.37; Mann-Whitney-U-test; P = 0.003) (Fig. 3B).