The 78th Symposium volume covers many aspects of the immune system including the genetics, biochemistry, molecular and cell biology, and developmental biology of immune responses. New approaches intended to harness the immune system to treat disease, particularly cancer, are also discussed. The volume's broad synthesis of the current knowledge about immunity and tolerance includes molecular mechanisms of B and T lymphocyte development from the single cell to the entire organism and from single genes to genomes. There is a focus on the development and function of innate cells, including myeloid cells, natural killer cells, and a more recently defined innate lymphoid cell that resembles a T cell. Transcriptional regulation of key immune pathways in innate and adaptive cells of the immune system are covered. The effect of metabolic pathways on immune responses is discussed and, conversely, how immune cells may affect cell and tissue physiology outside of the immune response. Other topics include the protective effect of commensal bacteria, the diverse immune mechanisms underlying chronic infection or immune control in HIV and tuberculosis, and insights into autoimmune diseases that may lead to new clinical therapies. Specific coverage includes: Stem cells and cell fate decisions Regulation of immune cell development Antigen receptor gene assembly and modification Signal transduction Regulation of lymphocyte function Innate immune response and inflammation Adaptive immunity Mucosal immunity Organ specific immunity Immune regulation and tolerance Autoimmunity and allergy Immunity and cancer Pathogen-immune system interactions Vaccine development Novel strategies to engineer/harness immunit