07 antigenic variation

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Antigenic Variation Introduction Antigenic Variation is the mechanism wherein infectious organisms such as viruses and bacteria alter their surface proteins to disguise and evade the immune reaction generated by the white blood cells. Mostly the organisms that attack old-aged people often target the same people and are easily transmittable. Antigenic Variation helps the pathogen dodge the immune system and then assist re-infection by the microbes. This happens because the microbes’ antigens are no longer identifiable by the immune system of the host.

Mechanism of the Immune system Usually when an antigen is introduced into the human body, the immune system generates an immune reaction and stimulates antibodies to attack that antigen. The body’s immune system is then familiarized with the particular antigen and the immune system acquires the immune response against it. In case the same pathogen attacks again, the antibodies will react faster to destroy the pathogen.

Mechanism of the Antigenic Variation Meanwhile, in case the pathogen is capable of antigenic variation it can dodge the host’s immune defenses. The immune system has to regenerate new antibodies for the new pathogen and hence, the antigen buys more time for the attack. In such cases, the antigens usually alter their surface by interchanging the arrangement of the proteins and carbohydrate molecules on their external membranes. There are other mechanisms to antigenic variations as well. These include gene conversion, DNA inversions, hypermutation and the combination of cassette sequences.

Antigenic Variation in Bacteria To become diverse, bacteria can go through antigenic as well as phase variation. Bacteria often variate their protein polymers which are also called Pilin. Pilin play an essential role in the generation of an immune response.

Antigenic Variation in Protozoa To discuss the antigenic variation in protozoa, let us consider the example of Trypanosoma brucei. Trypanosoma brucei is a protozoan that makes the host sleep sick. It regenerates in the bloodstreams of mammals. Because of its regeneration capability, it is instantly detected by the host’s immune system. To protect itself therefore, it processes antigenic variation by decorating Page 1

itself with a coating of glycoprotein. Initially, the coat protects the protozoan, but eventually it gets detected by the immune system.

Antigenic Variation in Viruses Viral infections are usually immediately cleaned by the immune defense. Some viruses such as Influenza may reoccur. It is because viruses produce virions, which resist the antibodies produced by the immune system. Influenza usually processes antigenic variation by generating a film (membrane) of peptide HA on its surface. When the virus is detected, the antibodies easily break the HA peptide into two sub-units, i.e. HA1 and HA2. The virus becomes stronger if the peptides at the cleavage sites are lipophilic.

Source: http://www.researchomatic.com/the-molecular-mechanism-of-antigenic-variation-155759.html

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