Antigenic shift is the process by which two or more different strains of a
virus
A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...
, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface
antigen
In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response.
...
s of the two or more original strains. The term is often applied specifically to
influenza
Influenza, commonly known as the flu, is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These sympto ...
, as that is the best-known example, but the process is also known to occur with other viruses, such as
visna virus in sheep. Antigenic shift is a specific case of
reassortment
Reassortment is the mixing of the genetic material of a species into new combinations in different individuals. The product of reassortment is called a reassortant. It is particularly used when two similar viruses that are infecting the same cell ...
or viral shift that confers a
phenotypic change.
Antigenic shift is contrasted with
antigenic drift, which is the natural
mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...
over time of known strains of influenza (or other things, in a more general sense) which may lead to a loss of immunity, or in vaccine mismatch. Antigenic drift occurs in all types of influenza including
influenza A,
influenza B and
influenza C. Antigenic shift, however, occurs only in influenza A because it infects more than just humans.
Affected species include other
mammal
A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s and
bird
Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s, giving influenza A the opportunity for a major reorganization of surface antigens. Influenza B and C principally infect humans, minimizing the chance that a
reassortment
Reassortment is the mixing of the genetic material of a species into new combinations in different individuals. The product of reassortment is called a reassortant. It is particularly used when two similar viruses that are infecting the same cell ...
will change its
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...
drastically.
In the 1940s,
Maurice Hilleman discovered antigenic shift, which is important for the emergence of new viral
pathogen
In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...
s as it is a pathway that viruses may follow to enter a new
niche.
Role in the transmission of influenza viruses from non-human animals to people
Influenza A viruses are found in many different animals, including ducks, chickens, pigs, humans, whales, horses, and seals.
Influenza B viruses circulate widely principally among humans, though it has recently been found in seals. Flu strains are named after their types of
hemagglutinin and
neuraminidase
Exo-α-sialidase (, sialidase, neuraminidase; systematic name acetylneuraminyl hydrolase) is a glycoside hydrolase that cleaves the glycosidic linkages of neuraminic acids:
: Hydrolysis of α-(2→3)-, α-(2→6)-, α-(2→8)- glycosidic linkag ...
surface
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
s (of which there are 18 and 9 respectively), so they will be called, for example, H3N2 for type-3 hemagglutinin and type-2 neuraminidase. Some strains of avian influenza (from which all other strains of influenza A are believed to stem
) can infect pigs or other mammalian hosts. When two different strains of influenza infect the same cell simultaneously, their protein
capsid
A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or m ...
s and
lipid
Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...
envelopes are removed, exposing their
RNA
Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...
, which is then transcribed to
mRNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein.
mRNA is ...
. The host cell then forms new viruses that combine their antigens; for example, H3N2 and H5N1 can form H5N2 this way. Because the human
immune system
The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...
has difficulty recognizing the new influenza strain, it may be highly dangerous, and result in a new pandemic.
Influenza viruses which have undergone antigenic shift have caused the
Asian Flu pandemic of 1957, the
Hong Kong Flu pandemic of 1968, and the
Swine Flu scare of 1976. Until recently, such combinations were believed to have caused the infamous
Spanish flu outbreak of 1918 which killed 40~100 million people worldwide. However, more recent research suggests the 1918 pandemic was caused by the
antigenic drift of a fully avian virus to a form that could infect humans efficiently. The most recent 2009 H1N1 outbreak was a result of antigenic shift and reassortment between human, avian, and swine viruses.
Role of pigs in Influenza antigenic shift
Pigs are especially important in antigenic shift of influenza viruses. Because pigs can be infected with strains of influenza that infect various other species of animals, they act as 'mixing pots' for the virus. When multiple virus strains, such as a duck and human influenza strain, infect the same pig, antigenic shift is likely to occur. While most of the virus strains resulting from this will be dead-end strains, a few have the potential to become pandemic viruses.
See also
*
Coinfection
*
Antigenic variation
Notes
Further reading
*
External links
Superflu: Antigenic shift in Influenza
{{DEFAULTSORT:Antigenic Shift
Virology
Influenza