Immune System
The immune system has a difficult time recognizing cancer cells. Equipped with a powerful immune system consisting of an innate arm and an adaptive arm, the human body is, however, capable of combating invaders for self-protection. While it is more straightforward for the immune system to recognize species and associated substances and markers that are clearly “foreign”, when it comes to the recognition of cancer cells, it is actually a difficult call on the part of immune cells. The chief challenge lies in the very fact that cancer cells were once normal "self" cells that the immune cells were vigorously trained not to attack. Recall that our T lymphocytes (T cells) originate in the bone marrow and become “educated” in the thymus where they learn to distinguish "self" cells from "non-self" cells. Since cancer cells are normal cells that have gone awry, many of the surface antigens are shared by both normal cells and cancer cells. Accordingly, it is very a tough judgment call on the part of T cells and natural killer (NK) cells to precisely recognize and pinpoint cancer cells for destruction.
Cellular Mutation
Cancer cells undergo constant mutation. As an added layer of complexity, cancer cells also undergo mutation in response to treatment, whether chemotherapy, targeted therapy, or immunotherapy. These resultant mutations help disguise cancer cells as "not foreign" to evade immune surveillance and escape immune destruction. Cancer cells are very adept at these molecular tricks which also explains why it is a challenging task to make cancer remit and then sustain remission.
