In this study, we report that repeated (greater than 5–6 injections over time) intraperitoneal administration of a commonly used hamster anti-mouse anti-PD-1 (αPD-1) (clone: J43) monoclonal antibody caused fatal hypersensitivity reactions in the highly inflammatory orthotopic 4T1 murine mammary carcinoma model. This evidence concerns the gene PDCD1 and breast carcinoma.