The ABCs of Flu

Doctors divide flu viruses into three broad categories or types: A, B or C. The letters refer to major differences in the virus’s nucleoprotein—the complex of protein and nucleic acid that contains the virus’s genetic blueprint.

The seasonal flu strain comes from either type A or B, both of which can cause epidemics.
Type A viruses are also found in birds, pigs and many other animals.
Type B circulates only in humans.
Type C influenza typically causes mild or no symptoms.

Both A and B types may cause an epidemic, but the severity is diminished in type B. Research currently shows that only type A flu viruses can cause pandemics.

Type A viruses are further identified by various protein spikes that dot the virus’s outer covering. Researchers have identified 16 different hemagglutinin or H proteins and 9 different neuraminidase or N proteins. You will also see the H and N proteins referred to as antigens which are substances that stimulate the immune system to produce antibodies. (Other common antigens are found on bacteria, pollen, dog and cat hair, etc.)

The H antigens govern the ability of the virus to bind to and enter cells, where multiplication of the virus then occurs. The N antigens govern the release of newly formed viruses from the cells.

For the past several years, H1N1, H3N2 and H1N2 have been the most common flu subtypes in people.

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