How flu viruses beat the immune system

Influenza virusesMicrobes that are only able to multiply within living cells. have the ability to cause epidemics, and sometimes may be responsible for a pandemicAn outbreak of infection that affects numerous people in different countries. .[1] To understand why, we need first to understand how the immune systemThe organs specialised to fight infection. works.

Infection with virusesMicrobes that are only able to multiply within living cells. or bacteriaA group of organisms too small to be seen with the naked eye, which are usually made up of just a single cell. triggers the immune systemThe organs specialised to fight infection. to release proteins called antibodiesSpecial proteins in the blood that are produced in response to a specific antigen and play a key role in immunity and allergy.. The specific part of the virusA microbe that is only able to multiply within living cells. or bacterium that triggers the antibodyOne of a group of special proteins in the blood that are produced in response to a specific antigen and play a key role in immunity and allergy. response is called an antigenA substance that prompts the immune system to fight infection with antibodies..

Once produced by the body (specifically, by the lymphoid tissuePart of the body’s immune system responsible for antibody production and the production of a type of white blood cells, lymphocytes., which is part of the body's immune systemThe organs specialised to fight infection.), antibodiesSpecial proteins in the blood that are produced in response to a specific antigen and play a key role in immunity and allergy. circulate in the bloodstream until they come into contact with an antigenA substance that prompts the immune system to fight infection with antibodies.. The antibodiesSpecial proteins in the blood that are produced in response to a specific antigen and play a key role in immunity and allergy. can either trigger other elements of the body's immune systemThe organs specialised to fight infection. to act on the antigenA substance that prompts the immune system to fight infection with antibodies., or they can act on the antigenA substance that prompts the immune system to fight infection with antibodies. directly to render it harmless.

The key to the potential of influenzaA viral infection affecting the respiratory system. virusesMicrobes that are only able to multiply within living cells. to spread so easily lies in their ability to evade the human immune systemThe organs specialised to fight infection.. They do this by changing their antigens, so that our antibodiesSpecial proteins in the blood that are produced in response to a specific antigen and play a key role in immunity and allergy. cannot recognise them and therefore fail to trigger the body's immune defences.

When the virusesMicrobes that are only able to multiply within living cells. replicate (reproduce), changes take place in their genes - these changes are called mutations. Sometimes, this means that their antigens change. If a number of mutations take place, the virusA microbe that is only able to multiply within living cells. antigens may change so much that our antibodiesSpecial proteins in the blood that are produced in response to a specific antigen and play a key role in immunity and allergy. no longer recognise them.[2]

Less often, changes in virusA microbe that is only able to multiply within living cells. antigens may happen as a result of two influenzaA viral infection affecting the respiratory system. virusesMicrobes that are only able to multiply within living cells. infecting the same animal. The two virusesMicrobes that are only able to multiply within living cells. combine, creating a new virusA microbe that is only able to multiply within living cells. that has elements of each of the original virusesMicrobes that are only able to multiply within living cells.. This can bring about a rapid, drastic change in the viral antigens rather than the slow, gradual changes more usually seen. [2]

References: 
  1. DM Fleming. Influenza diagnosisThe process of determining which condition a patient may have. and treatment: A view from clinical practice. .
  2. Yamshchikov AV, Desai NS, Blumberg HM, Ziegler TR, Tangpricha V.
    Vitamin D for treatment and prevention of infectious diseases: a systematic review of randomized controlled trials. Endocr Pract. 2009 Jul-Aug;15(5):438-49.