This document provides an overview of immunotherapy and how it works to treat cancer. It discusses the roles of T cells, antigen presenting cells, and cytokines in the immune response. It describes how tumors evade immune surveillance and strategies used in immunotherapy to overcome tumor resistance, such as blocking inhibitory receptors like CTLA-4 and PD-1 to reactivate T cells. Several studies are summarized that show improved survival outcomes for cancers like melanoma when treated with immunotherapies like nivolumab compared to chemotherapy. Combination approaches blocking multiple pathways are also discussed.