What is antigenic variation and how is it beneficial to pathogens? give an example of two pathogens that demonstrate antigenic variation.

Antigenic variation is the change occurs in antigenic property of the pathogen. In antigenic variation, a pathogen does changes in its surface protein or carbohydrate so that the immune system of the host is not able to recognize the pathogen.  

This antigenic variation is very much beneficial to pathogens because it gives advantage to the pathogen to infect the host again and again without being recognized by immune cells and to increase in number.

A recombination of avian and human influenza virus in a host like pig created a new version of the influenza virus which became very endemic worldwide is a classic example of antigenic shift. M. tuberculosis evades immune response by doing antigenic shift it's membrane and cause tuberculosis.

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