CTLA4 and anaphylaxis: It has previously been reported by our group that 4T1 tumor-bearing mice will succumb to anaphylaxis after the repeated administration of xenogeneic hamster or rat αPD-1 monotherapy but not with xenogeneic species-specific antibody controls or xenogeneic anti- cytotoxic T-lymphocyte-associated protein 4 (CTLA4) mAb [18].