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In humans and domesticated animals, the immune system is involved in a large proportion of all known diseases, which has lead to an increased understanding of this immensely complex system and the introduction of therapies that modify the immune response.
In multicellular organisms, the immune system is an organ system that acts as a defense against foreign pathogens (such as viruses, bacteria, parasites), some poisons, as well as cancer. Components of the immune system also function in the return of extracellular fluid to the blood, and the formation of white blood cells.
Bacteria and monocellular organisms have an "immune system" designed to combat bacteriophages (viruses that infect bacteria). They do this by simultaneously expressing enzymes that cut DNA at certain sequences, and enzymes that protect DNA from this enzyme by methylating the same sequence. Therefore, the bacterium's DNA will not be damaged by the first enzyme because of the presence of the second enzyme. However, when a bacteriophage attempts to infect this bacterium, the viral DNA has not been protected, and gets degraded by the first enzyme.
However, when we talk about immune systems we are usually referring to the immune systems of multicellular organisms, usually vertebrates.
The Latin term immunis means exempt, referring to protection against foreign agents. The recognition of what is foreign is found in all life. In self-pollinating plants, a pollen grain landing on the stigma of a flower will send a pollen tubule down the style to the ovary for fertilization. A pollen grain from a genetically distinct plant will not germinate or the pollen tubule, once formed, will disintegrate in the style. In cross-pollinating species, self-marked pollen grains disintegrate, while nonself grains germinate and fertilize.
We may conceive of an arrangement where the cells of self are marked, so that they are not attacked by its own defense mechanism. But not all foreign cells may be destroyed since some must be assimilated for nourishment. Therefore, the immune system must have the capacity to detect self and some nonself. But since self needs to assimilate some nonself for its survival, it cannot mark itself. It is easier to mark potentially dangerous selves. But if only certain nonselves are marked, how does the body prepare to defend itself from selves not seen? The defense system must have the capacity to transform itself to deal with future dangers. It must also have the capacity to change, since the self itself evolves with time. An additional challenge to understanding is the mechanism by which sexually reproducing organisms prevent the growing embryo from being destroyed by the immune system of the 'mother'. It is believed that this is achieved by the specialized tissue such as the placentum in placental mammals. New theories attempt to solve some of these paradoxes. One such is the 'danger theory' proposed by Polly Matzinger which suggests that cellular apoptosis signals and directs the immune mechanism. Another is the 'Pathogen-Associated Molecular Pattern' theory proposed by Charles Janeway which suggested that conserved molecular patterns found on pathogens provide a context in which a particular antigen is recognized.
Most multicellular organisms possess an immune system consisting of innate immunity which generally consists of a set of genetically-encoded responses to pathogens and does not change during the lifetime of the organism. Adaptive immunity, in which the response to pathogens changes during the lifetime of an individual, appeared somewhat abruptly in evolutionary time with the appearance of cartilaginous (jawed) fishsee text The Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nostrils, scales, two-chambered hearts, and skeletons made of cartilage. They consist of several orders: Subclass Elasmobranchii ( sharks, rays and skates) Superor. Organisms that possess an adaptive immunity also possess an innate immunity and many of the mechanisms between the systems are common, so it not always possible to draw a hard and fast boundary between the individual components involved in each, despite the clear difference in operation. Higher vertebrates and all mammalSubclass Monotremata Monotremata Subclass Marsupialia Didelphimorphia Paucituberculata Microbiotheria Dasyuromorphia Peramelemorphia Notoryctemorphia Diprotodontia Subclass Placentalia Xenarthra Dermoptera Desmostylia Scandentia Primates Rodentia Lagomorps have both an innate and an adaptive immune system.