The immune
system recognizes viruses when the antigen on the surface ofthe virus articles
binds to the immune receptors that are specific for these antigens; this is
similar to a lock recognizing a key.
After an
infection, the body produces more of these virus specific receptors which
prevent re-infection by these particular strains of virus and produce acquired
immunity.
Similarly,
a vaccine against a virus works by teaching the immune system to recognize the
antigen exhibited by the virus. However, viral genome are constantly mutating,
producing new forms of an antigen in sufficiently different forms, old antigen
will no longer bind to the receptors and virus with these new antigen will
evade immunity to the original strain of the virus. When such a change takes
place, people who had the illness in the past will lose their immunity to the
new strain and vaccine against the original virus will lose its effectiveness
Two processes
causes the antigen to change,
a- Antigen
Drift:-This is
the random accumulation of mutants in the viral gene recognized by the immune
system. Such accumulation may significantly change the antigen of the virus and
the antigen may help it evade the immune system.
b-Antigen
shift: - This
is the genetic change that enable a viral strain to jump from one host to the
other, in other words, from one animal
species to another including human. It is also the process by which two strains
of two or more different viruses combines to form a sub type having a mixture
of the surface antigen of the two or more original strains.
The term
applies specifically to influenza as it remains the best known example,
although the process also occurs with other viruses such as VISINA virus in
sheep.
Antigen
shift is a specific case of assortment or viral shift that confirms a phenotype
change.
The
process occur in all types of influenza, including type A, B and C. However, it
occur more in influenza type A, because it infest more than just human. Type A
infects other species including mammals and birds, giving type A the
opportunity for a major reorganization of surface antigen.
Influenza
type B and C, mainly infect human minimizing the chance that a reassortment
will drastically change its phenotype.
Antigen
shift is important for the emergence of a new viral pathogen, it’s a path that
virus may take to enter a new host. It
could occur with primate virus and may be a factor for the appearance of a new
virus in human such as HIV. Due to the structure of the genome, HIV does not
undergo reassoertment but does recombine freely and via super infection. It can
produce recombinant HIV strains that differ significantly from their ancestors.
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