Both drugs turn off a natural brake on immune system activity called CTLA4 (cytotoxic T-lymphocyte antigen 4).

FORBES: Magazine Article

Pfizer's drug tremelimumab and Bristol-Myers Squibb's ipilimumab are antibodies to a protein called CTLA-4 (cytotoxic T-lymphocyte antigen-4) that acts as an emergency brake to prevent killer T cells from attacking healthy tissue.

FORBES: Targeting Melanoma