An immune response targeted against our own cells

An important function of our immune system is determining our own cells from foreign microorganisms. When is it beneficial to have an immune response targeted against our own cells? 

Cancer is caused by the un-regulated proliferation of our own cells and results in serious damage and disease. An immune response directed against cancerous cells is beneficial, as immune cells can directly kill cancerous cells. Therapies boosting anti-cancer immune responses have shown great promise.