Immunology, the study of the body’s immune system. It was originally the branch of medical science dealing with defence or resistance against infections, but the term has broadened over the past four decades to encompass all processes and mechanisms which discriminate between “self”—that is, the body’s own innate mechanisms, molecules, cells, and tissues, and everything that belongs to it—and “non-self”—anything which comes from outside the body; that which is alien to it. The latter term includes infectious micro-organisms (protozoa, fungi, bacteria, mycoplasmas, and viruses); parasites; toxins and poisons of sufficient size and appropriate composition, tumours and neoplastic cells; transplants; and transfused cells or molecules from genetically non-identical animals.
THE IMMUNE RESPONSE
Most animals are capable of mounting a defensive response against non-self substances; this is known as the immune response. The study of the natural development of mechanisms involved in the immune response is the main feature of immunology and immunological research. Immune responses can be classified as innate (meaning those which occur without prior exposure to the foreign substance, organism, or tissue) or acquired (meaning those which require exposure to the non-self material).