WJEC Biology for A2: Student Bk

Examtip Remember that antigenic drift produces gradual changes and the lack of adequate immune response means a new vaccine is needed each year. Antigenic shift is a recombination of H and N types and can cause epidemics. ▪ The influenza virus attacks mucous membranes, especially in the upper respiratory tract, causing sore throat, cough and fever. ▪ The spread of ’flu virus is not easy to control. It is inhaled in droplets from coughs and sneezes, in aerosol transmission. Mucus protects the virus. It also survives better when the air is dry and there is low ultra-violet light in the environment, which means it survives better in winter than in summer, giving rise to seasonal ’flu. ▪ There are several ways to reduce the risk of infection: – Regular hand washing. – Using and discarding tissues for coughs and sneezes. – Influenza vaccines can be effective, but are variable in their success. The surface antigens on the virus change and so a new vaccine is needed annually. – Quarantine. Antigenic types There are many antigenic types of influenza viruses. Their differences have two main origins: 1. Antigenic drift There are no RNA proofreading enzymes, so following each round of replication, on average, every new virion has a new mutation. This produces a gradual change in the surface proteins, which is called antigenic drift. It explains why, each year, a new vaccine is needed. 2. Antigenic shift ’Flu A has 16 different types of haemagglutinin, of which H1, H2 and H3 are the most common in humans. It has nine different types of neuraminidase, of which N1 and N2 are commonest in humans. If one cell is infected by viruses that have different combinations of H and N, e.g. H1N2 and H2N1, the separate strands of RNA can recombine, giving rise to new virus types, e.g. H1N1 or H2N2. This change is ‘antigenic shift’ and the new virus types can cause epidemics . Antigenic shift: the formation of a new flu virus type by recombination In some parts of the world, people live in close contact with animals. Chickens and pigs, in particular, are an animal reservoir for new virus infection in humans, i.e. a source of a new infection produced when an animal virus mutates and becomes able to infect humans. The influenza strains that infect people worldwide are continually monitored; H7N9 has been of particular concern since 2013. The table shows the H and N combinations in some recent influenza epidemics. There are more details of the immune response and vaccination on p274. Link Key Terms Antigenic type : Different individuals of the same pathogenic species with different surface proteins, generating different antibodies. Epidemic : The rapid spread of infectious disease to a large number of people within a short period of time. RECOMBINATION H1 N2 H5 N1 H1 N1 new virus strain by recombination 2 virus strains infect WJEC A2 Biology: Chapter 14 264