Immunology is the study of the immune system, the structures and processes an organism uses to protect against and fight disease. The simplest of organisms, bacteria, possess rudimentary enzymatic defenses against pathogens. More complicated organisms, such as plants and insects, possess additional defense mechanisms such as phagocytosis. Vertebrates have still more complicated immune systems, involving layers of defense mechanisms, from general barriers and innate, non-specific mechanisms, to an adaptive immune system with highly specific responses to individual pathogens and mechanisms of immunological memory. Immunology as a field of study dates back to the time of the plague in Athens in 430 BC. It was observed by Thucydides that plague survivors did not contract the disease a second time (were immune) and so could safely nurse the sick. It was not until the late 19th century that the germ theory of disease was accepted. The distinctions between humoral immunity and cellular immunity became better understood in the late 19th and early 20th century. The advent of molecular biology allowed for the development of the field into one of the most promising areas of biology research.

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