New influenza viruses jump from animals to humans with alarming frequency, as evidenced by the H5N1 bird flu virus or, more recently, with the swine flu virus. Although humans usually do not have any preexisting immunity to such pathogens, the human body is not completely unprotected against the invaders. It can rapidly mobilize a defense strategy which prevents the influenza viruses from proliferating unchecked in the body.
An essential element of this protection is a protein produced by the body which recognizes many viruses and prevents them from replicating inside infected cells. Under normal conditions this protective protein is not present in the cell at all, but after infection it can be produced in large quantities. The order to produce this protein is made by the signaling protein interferon, which is excreted by infected cells and alarms the organism of the virus infection.
The protein, known as Mx (short for myxovirus resistance), is a molecular machine which does not develop its full power until the individual molecules have joined to form a ring-structured macromolecular network. A central element of the formation of these ring structures is the special part of Mx known as the stalk.
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