Influenza (more simply, “the flu”) is caused by certain Orthomyxoviridae RNA viruses, which are categorized as influenza Types-A, B, and C. Of the three types of Influenza, Type A and Type B produce serious illness each year. Viruses change or mutate, and Type A is by far the fastest in doing so. Type C produces milder sickness (respiratory) and does not appear to be associated with epidemics. Influenza particles are largely genetic material encased by specific proteins that form an outer surface coat called a “capsid.” The Influenza A capsid incorporates, most notably, hemagglutinin, H (the main surface glycoprotein) and neuraminidase, N.
Antigens and Subtypes
The numbers following the letters define influenza subtypes. H5N1, for instance, constitutes a different subtype-the Avian Influenza or “bird flu.” The H5 and the N1 are specific antigens. An antigen is defined as antigen is defined as “any substance that stimulates an immune response.” Thus each differently numbered hemagglutinin is a separate substance that stimulates an immune response. To date, there are 16 known H antigens and 9 N antigens. As in the case of Influenza A, Influenza B has a protein coat consisting of hemagglutinin and neuraminidase, but it is not divided into subtypes.
Beyond types and subtypes, Influenza viruses are further delineated by strains caused by mutation . Type A is by far the fastest mutating type. Type B is two or three times slower, and C is slower still. It is this mutation that makes viruses so potentially dangerous. For instance, of the H1N1 subtype there are a number of strains. Although the current strain, known as Swine Flu, represents a threat, to date it has not been the killer a former strain of H1N1 was. That strain-the Spanish Influenza of 1918-was responsible for the loss of up to 50 million lives worldwide!
Strain Changing Mechanisms
What form does this mutation take? There are two forms. One is called antigenic shift-the other antigenic drift. Antigenic shift involves the interaction of two different strains of a virus to produce a new virus subtype (not merely a strain). This is limited to Influenza Type A. It can be extremely dangerous or not, depending upon the presence or absence of two factors. 1
1. If the new subtype is a form harmful to humans, and
2. If the new subtype is readily spread in a sustained manner.
The other form of strain mutation is similarly named, Antigenic Drift. Antigenic drift is more gradual and involves the genetic material beneath the capsid. That material alters the capsid, and the body of the victim may not recognize the new virus particle which are thus viewed as new to the body’s immune system. It is this latter mechanism that forces the scientific community to develop new vaccines on a yearly basis.
1 Centers for Disease Control and Prevention – Influenza Viruses
References and Resources:
Planet Flu – The Influenza Virus Gallery
The Why Files – Flu Season-What is Flu?