The body produces millions of antibodies, each uniquely designed to target specific pathogens: bacteria, viruses, and other parasitic invaders. Antibodies recognize and bind the pathogen, send chemical signals to other immune cells, and mark the pathogen as ready for destruction. Dead man walking!
When it comes to cancer, the body has a little trouble targeting infected cells because they are in some ways still the body's own cells. Fortunately, the immune system does have a technique in place to recognize irregular cells, so cancer cells can also be marked for destruction. Specific immune cells, called cytotoxic T lymphocytes (CTLs) induce self-destruction in abnormal, cancerous cells. In fact, in the 1970s, scientists began mass-producing antibodies to boost the immune response.
The immune system also has a control mechanism in place to ensure CTLs don't become "overly enthusiastic" and contribute to autoimmune reactions where healthy cells are destroyed. However, cancer cells take advantage of the "off switch" to prevent autoimmunity, pulling the switch to prevent destruction by CTLS.
Scientists developed monoclonal antibodies to solve this problem, either by directly blocking the cancer cell from activating the off switch or by binding to the CTL to prevent the cancer cell from being able to bind. Either way, the science of monoclonal antibodies ensures that CTLs can do their job effectively without causing an autoimmune response, and cancer doesn't stand a chance.