Utomilumab is a monoclonal antibody that binds to 4-IBB protein (also known as CD137 protein). 4-IBB protein is a receptor found in different immune cells such as helper T-cells, killer T-cells and natural killer cells. 4-IBB/CD137 protein is a costimulatory receptor whose activation results in a series of events that leads to cytokine secretion and effector functions. Hence, targeting CD137 protein has been linked to antitumor immunity response and overall tumor reduction. Indeed, once utomilumab is injected in the bloodstream its specific binding to CD137 protein stimulates the immune cells which, as a result, enhances the immune response against tumors.