Cancer can mask itself from the immune system by hiding neoantigens or mutations that do not produce detectable antigens.

“what makes cancer so challenging for the immune system is that it's basically self and um it's very good at masking it's non-self entities so even though it's genetically distinct from a non-cancer cell it's very good at hiding uh the neoantigens that come you know and sometimes they're not even coding proteins in other words sometimes the mutation in the cancer cell doesn't lead to the generation of an antigen and therefore it can go completely undetected masquerading itself”