Cancer immunotherapy harnesses the immune system by targeting
tumor cells that express
antigens recognized by immune system cells, thus leading to
tumor rejection. These
tumor-associated
antigens include
tumor-specific shared
antigens, differentiation antigens,
protein products of mutated genes and rearrangements unique to
tumor cells, overexpressed tissue-specific
antigens, and exogenous
viral proteins. However, the development of effective therapeutic approaches has proven difficult, mainly because these
tumor antigens are shielded, and cells primarily express self-derived
antigens. Despite innovative and notable advances in
immunotherapy, challenges associated with variable patient response rates and efficacy on select
tumors minimize the overall effectiveness of
immunotherapy. Variations observed in response rates to
immunotherapy are due to multiple factors, including adaptative resistance, competency, and a diversity of individual immune systems, including cancer stem cells in the tumor microenvironment, composition of the gut microbiota, and broad limitations of current immunotherapeutic approaches. New approaches are positioned to improve the immune response and increase the efficacy of
immunotherapies, highlighting the challenges that the current global
COVID-19 pandemic places on the present state of
immunotherapy.