<P> Immunology is a science that examines the structure and function of the immune system . It originates from medicine and early studies on the causes of immunity to disease . The earliest known reference to immunity was during the plague of Athens in 430 BC . Thucydides noted that people who had recovered from a previous bout of the disease could nurse the sick without contracting the illness a second time . In the 18th century, Pierre - Louis Moreau de Maupertuis made experiments with scorpion venom and observed that certain dogs and mice were immune to this venom . This and other observations of acquired immunity were later exploited by Louis Pasteur in his development of vaccination and his proposed germ theory of disease . Pasteur's theory was in direct opposition to contemporary theories of disease, such as the miasma theory . It was not until Robert Koch's 1891 proofs, for which he was awarded a Nobel Prize in 1905, that microorganisms were confirmed as the cause of infectious disease . Viruses were confirmed as human pathogens in 1901, with the discovery of the yellow fever virus by Walter Reed . </P> <P> Immunology made a great advance towards the end of the 19th century, through rapid developments, in the study of humoral immunity and cellular immunity . Particularly important was the work of Paul Ehrlich, who proposed the side - chain theory to explain the specificity of the antigen - antibody reaction; his contributions to the understanding of humoral immunity were recognized by the award of a Nobel Prize in 1908, which was jointly awarded to the founder of cellular immunology, Elie Metchnikoff . </P> <P> The immune system protects organisms from infection with layered defenses of increasing specificity . In simple terms, physical barriers prevent pathogens such as bacteria and viruses from entering the organism . If a pathogen breaches these barriers, the innate immune system provides an immediate, but non-specific response . Innate immune systems are found in all plants and animals . If pathogens successfully evade the innate response, vertebrates possess a second layer of protection, the adaptive immune system, which is activated by the innate response . Here, the immune system adapts its response during an infection to improve its recognition of the pathogen . This improved response is then retained after the pathogen has been eliminated, in the form of an immunological memory, and allows the adaptive immune system to mount faster and stronger attacks each time this pathogen is encountered . </P> <Table> Components of the immune system <Tr> <Th> Innate immune system </Th> <Th> Adaptive immune system </Th> </Tr> <Tr> <Td> Response is non-specific </Td> <Td> Pathogen and antigen specific response </Td> </Tr> <Tr> <Td> Exposure leads to immediate maximal response </Td> <Td> Lag time between exposure and maximal response </Td> </Tr> <Tr> <Td> Cell - mediated and humoral components </Td> <Td> Cell - mediated and humoral components </Td> </Tr> <Tr> <Td> No immunological memory </Td> <Td> Exposure leads to immunological memory </Td> </Tr> <Tr> <Td> Found in nearly all forms of life </Td> <Td> Found only in jawed vertebrates </Td> </Tr> </Table>

Two physical barriers of the human body to infection
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