Cancer comprises many different diseases (estimated at more than 200 types of malignancies), all characterized by accumulation of DNA damage and uncontrolled proliferation of abnormal cells with capacity for spread to healthy organs and form malignant tumours. The therapeutic approaches in the last decades included surgery, radiation, chemotherapy and other advanced medical strategies. The scientists who contributed to these discoveries and improved therapeutic approaches to cancer treatment have been awarded Nobel prizes in the past. In 2018 the Nobel Prize in Physiology or Medicine was awarded to an American and a Japanese scientist for their discovery of a revolutionary approach to cancer treatment. Their revolutionary discovery proved that the human body’s immune system can be harnessed to attack cancer cells 20 years ago. It was known for many years before that the human immune system under normal physiological conditions seeks out and destroys mutated cells, but cancer cells find sophisticated ways to hide from immune attacks, allowing them to thrive and grow. Many types of cancer do this by ramping up a braking mechanism that keeps immune cells in check. In 2011, new anticancer drugs have been approved by the Food and Drug Administration (FDA, USA) as a therapeutic antibody drug ipilimumab (trade name Yervoy® Bristol-Myers Squibb Co) which is a monoclonal antibody that works to activate the immune system by targeting CTLA-4 (T-lymphocyte-associated protein). Also, in 2014 FDA approved PD-1 inhibitor pembrolizumab (trade name Keytruda®, Merck Sharp & Dohme Corp.) which is used in cancer immunotherapy. It is an IgG4 isotype antibody that blocks a protective mechanism of cancer cells, and allows the immune system to destroy those cancer cells. Prof. J. Allison’s research was fundamental. It focused on the question of how T cells worked in the fight against cancer cells because this basic knowledge was instrumental in developing the therapeutic aspects of immuno therapy. Prof. Tasuku Honjo (Department of Immunology at Kyoto University, Japan) and co-researchers discovered (1992) another protein PD-1 which functions in a similar way blocking the anticancer function of the immune system’s T cells. Prof. Honjo and his co-workers performed research to unravel PD-1 role. Meticulously explored its function in elegant experiments performed over many years. The results showed that PD-1, similar to CTLA-4, functions as a T cell brake, but operates by a different mechanism. The 2018 Nobel prize was a justified reward for professors J. Allison and T. Honjo for their revolutionary approach to cancer treatment by using the capacity of the immune system. This review describes the process of the discovery and the drugs that were approved for the immunotherapy trteatment of cancer. The discovery of immunotherapy has become the fourth pillar of cancer treatment via the adaptive immune responses..