PDCD1 and neoplasm: For an anti-PD1 antibody of IgG4 backbone, S228P mutation did not seem to completely prevent Fc-Fc interation and it was found to inhibit anti-tumor humoral immune response by Fc-Fc interaction with endogenous tumor-specific IgG1 or IgG4, which even promote tumor growth and result in hyperprogressive disease (17).