Until recently, immunology was not a major focus of attention in the acute and critical care setting. With the evolution of immunotherapy, however—including the development of monoclonal antibodies, checkpoint inhibitors, and adoptive cellular therapies—an in-depth understanding of the immune system has become necessary to properly care for acutely ill patients. Lymphocytes (ie, T cells and B cells) play a major role in the daily functioning of the immune response. The interaction of these cells in protecting the body against infection and foreign substances is complicated and is the basis for many of the innovations in immunology and cancer treatment. This article explores basic immunology concepts, new drug categories, and adoptive cellular therapies that are at the forefront of cancer therapy.