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.