Immunology is the branch of biology that studies the immune system, a complex network of cells, tissues, and organs that work in concert to defend an organism against pathogens like bacteria, viruses, and fungi. Central to this field is understanding how the body distinguishes its own healthy "self" cells from foreign "non-self" invaders or altered "self" cells, such as cancerous ones. It encompasses the study of both the innate (non-specific) and adaptive (specific) immune responses, as well as the investigation of immune system disorders, including autoimmune diseases, hypersensitivities, and immunodeficiencies, which provides the foundational knowledge for vaccination, organ transplantation, and immunotherapy.