Influenza has occurred throughout history, but the world became aware of its deadly potential in 1918-19 when a pandemic--a worldwide epidemic--seemed to strike out of nowhere. It killed some 40 million people--many more than the number killed in World War I. This pandemic arose from a bird flu virus that adapted to humans, an event that scientists fear could happen with H5N1.
Although widespread influenza pandemics did not erupt again until 1957 and 1968, there is evidence that a virus resembling the 1957 strain was circulating among humans as far back as 1888.
After the 1918-1919 pandemic, immunologists learned that the immune system responds to influenza A viruses in two basic ways. The first is to stimulate the B lymphocytes that develop into antibody-forming plasma cells. If a person has the "right" antibodies in his or her blood as a consequence of being vaccinated, that person is completely protected. On the other hand, the CD8+ "killer" T lymphocytes, which attack and kill cells infected by the virus, take longer to respond, and the virus still replicates extensively before the lymphocytes can do their job. Even before people realize they have been infected, the flu viruses multiply rapidly in the respiratory system and leap to nearby people in the fine droplets of coughs or sneezes. This explains why yearly human flu epidemics can seem to explode out of nowhere and spread rapidly through a household and community before fading away.
Moreover, the HA and NA proteins of these viruses continually mutate, keeping a step ahead of the posse of antibodies that seek to bring them down. This molecular strategy, which forces scientists to redesign the flu vaccine each year, is called antigenic drift. An antigen is a molecule that triggers an immune system attack.
In contrast, an antigenic shift occurs when different viruses infec
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Contact: Kelly Perry
media@stjude.org
901-495-3306
St. Jude Children's Research Hospital
4-May-2006