Antigenic shift and drift

1. Antigenic Shift & Antigenic Drift
Presented By
ABDULLAH
Roll no 03
M.phil Microbiology
Batch 03
Faculty of Life sciences
Department of Microbiology
Abdul Wali Khan University, Mardan
Pakistan

2. Antigenic Shift
• A sudden shift in the antigenicity of a virus resulting
from the recombination of the genomes of two viral
strains.
• The shift occurs when different strains of same virus or
strains of different viruses combine and form a new
subtype.
• The resulting new subtype having a mixture of surface
antigens of the two or more original strain.
• Only in influenza virus the antigenic shift is seen.
Specially influenza A virus.

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• Antigenic shift confers a phenotypic change by the
reassortment of viral genomes.
• New viral pathogens follows the pathway of antigenic shift
to get entry into the new environment thus antigenic shift
is important for the emergence of new viral pathogens.
• Influenza virus is the best known example of antigenic shift
as it mostly specifically applied to it, but the process of
antigenic shift may also occur in other viruses such as Visna
virus that cause encephalitis and chronic pneumonitis in
sheep.

4. Structure of Influenza Virus

5. Antigenic Shift in Influenza virus
• By the types of hemagglutinin and neuraminidase surface
proteins flu viruses are named. For example, H3N2 is
named that it have type-3 hemagglutinin and type-2
neuraminidase surface proteins.
• When human cell is infected by the flu virus such as H3N2
and H5N1 at the same time, the virus uncoated and
release its viral genome to the host cell and transcribed to
mRNA which is then translated to viral proteins and a new
virion is formed by assemblage.

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• Now in this case the host cell is infected by two different
strains of influenza virus H3N2 and H5N1 so the possibility is
present that it form a new viral strain H5N2.
• The H5N2 strain is new to the host which cause difficulties
in recognizing and illness. Transmission from person to
person can occurs and lead to epidemic or pandemic.

8. Pig model and Antigenic Shift
• In case of pigs which is infected by swine influenza and
susceptible to avian influenza cause infection in ducks and
human influenza virus. Infected pigs with these three types
of different viruses shows similar symptoms such as cough,
fever, and runny nose etc.
• During replication and recombination of viruses it is
possible that a new strain is formed by the different three
viral genome combinations.

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• For example, a pig was infected by the human and avian
influenza simultaneously. In this case antigenic shift could
occur, which lead to produce a new virus which probably
had most genes from human influenza virus and
hemagglutinin and neuramindase surface proteins.
• The resulting virus is new and cause infection in humans as
well as transmissible from person to person. The
population will not be immune to this new strain as it new
and thus no immune protection a pandemic may occur.

11. Antigenic Drift
• The mechanism of mutation and variation in viruses within
the genes that code for antibody-binding sites is known as
antigenic drift.
• The result of this mechanism or phenomena is a novel strain
of virus particles which cannot be inhibited by antibodies
effectively as they were originally inhibited in previous
strains of the same virus, by this mechanism the virus is
able to spread throughout the body and invade or may
escape the immune system.
• Antigenic drift occurs in both strains of influenza A & B
virus.

12. Antigenic drift in Influenza virus
• Hemagglutinin (HA) and neuraminidase (NA) are the two
relevant surface proteins antigens present in the influenza virus.
• The responsibility of hemagglutinin in the influenza virus is
binding and entry into host epithelial cells while on the other hand
neuraminidase is responsible in the process of budding out of new
virions budding from the host cells.
• Sites present on the hemagglutinin and neuraminidase surface
proteins are recognized by host immune cells are under constant
selective pressure.
• Due to the selective pressure antigenic drift allows the influenza
virus for evasion of these host immune cells by creating small
mutations in the genes coding for hemagglutinin and
neuraminidase that make the protein unrecognizable to
pre-existing host immunity.

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• So antigenic drift is thus described as continued process of
genetic and antigenic changes among the flu virus strains.
• Individuals that are immune or vaccinated in human populations,
exert selective pressure for single point mutations in the gene
coding for hemagglutinin protein that increase receptor binding
avidity, while those individuals who are not immune or
vaccinated exerting selective pressure for single point
mutations that decrease receptor binding avidity.
• The observed rapid evolution in the gene coding for
hemagglutinin proteins in influenza virus are due to above
dynamic selection pressures that facilitate it.

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• Specifically, in the HA1 domain of the hemagglutinin gene
18 specific codons have been identified as undergoing
positive selection to change their encoded amino acid.
• Thus vaccines that are able of broad protection against
heterovariant strains are needed against seasonal, epidemic
and pandemic influenza to cope or overcome the challenge
of antigenic drift.

16. Antigenic Drift in HIV
• Antigenic drift has also been reported in Retrovirus (HIV)
and rhinovirus which cause common cold.
• In case of HIV infection body immune system produce high
titer of broadly neutralizing antibodies (bNAbs) against
HIV.
• But however antigenic drift occurs in gp120 surface
glycoprotein of the human immune deficiency virus due
some selection pressure by which it escape from the
immune system.
• The bNAbs not invading the modified gp120 strain of HIV.

17. Hong Kong Flu (1968–1969)
• The Hong Kong Flu was a category 2 flu pandemic caused
by a strain of H3N2 descended from H2N2 by antigeni shift,
in which genes from multiple subtypes reassorted to form a
new virus.
• Killed an estimated one million people worldwide.
• Infected an estimated 500,000 Hong Kong residents
• In the United States, about 33,800 people died.

18. Antigenic Shift vs Antigenic Drift