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Antigenic variation
Antigenic variation refers to the mechanism by which an infectious agent such as a protozoan, bacterium or virus alters its surface proteins in order to evade a host immune response. It is related to phase variation. Immune evasion is particularly important for organisms that target long-lived hosts, repeatedly infect a single host and are easily transmittable. Antigenic variation not only enables immune evasion by the pathogen, but also allows the microbes to cause re-infection, as their antigens are no longer recognized by the host's immune system. When an organism is exposed to a particular antigen (i.e. a protein on the surface of a bacterium) an immune response is stimulated and antibodies are generated to target that specific antigen. The immune system will then "remember" that particular antigen, and defenses aimed at that antigen become part of the immune system’s acquired immune response. If the same pathogen tries to re-infect the same host the antibodies will act rapidly to target the pathogen for destruction. However, if the pathogen can alter its surface antigens, it can evade the host's acquired immune system. This will allow the pathogen to re-infect the host while the immune system generates new antibodies to target the newly identified antigen. Antigenic variation can occur by altering a variety of surface molecules including proteins and carbohydrates. There are many molecular mechanisms behind antigenic variation, including gene conversion, site-specific DNA inversions, hypermutation, as well as recombination of sequence cassettes. In all cases, antigenic variation and phase variation result in a heterogenic phenotype of a clonal population. Individual cells either express the phase-variable protein(s) or express one of multiple antigenic forms of the protein. This form of regulation has been identified mainly, but not exclusively, for a wide variety of surface structures in pathogens and is implicated as a virulence strategy.
==Antigenic Variation in Bacteria==
To generate intra-population diversity, some bacteria can produce variation by various methods such as antigenic or phase variation. Antigenic variation is the expression of various alternative forms of antigen on the cell surface. Whereas phase variation is the phenotypic switch that is usually reversible and is referred to as an ON-OFF switch. The outcome of either method of variation has a beneficial effect that can result in increased fitness, evasion strategies or environmental adaptation.
Antigenic variation in bacteria is best demonstrated by species of the genus ''Neisseria'' (most notably, ''Neisseria meningitidis'' and ''Neisseria gonorrhoeae'', the gonococcus); species of the genus ''Streptococcus'' and the Mycoplasma. The ''Neisseria'' species mentioned variate their pili (protein polymers made up of subunits called pilin which play a critical role in bacterial adhesion, they are antigens which stimulate a vigorous host immune response) and the Streptococci variate their M-protein.
Additionally, Lyme disease is caused by the bacterium ''Borrelia burgdorferi''. The surface lipoprotein VlsE can undergo recombination which results in antigenic diversity. The bacterium carries a plasmid that contains fifteen silent ''vls'' cassettes and one functional copy of ''vlsE''. Segments of the silent cassettes recombine with the vlsE gene. Variety generated of the surface lipoprotein antigen allows the bacterium to evade the host humoral immune system.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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