Two mating types of yeast cells are: 

1. a 

2. α

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Adenoviruses and cytomegalo virus use to reduce the surface expression of class I MHC molecules, 

Measles virus and HIV, reduce levels of class II MHC molecules on the surface, 

Vaccinia virus, secreting a protein that binds to the C4b complement component.

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Influenza virus

Influenza are surrounded by an outer envelope, a lipid bilayer derived from the plasma membrane of the infected cell.

Viral specific Protein 

Hemagglutinin (HA) 

Neuraminidase (NA)

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HA trimers are responsible for the attachment of the virus to host cells, binding to the sialic acid. 

NA is an enzyme that cleaves N-acetyl neuraminic (sialic) acid

Within the envelope, consists of eight different strands of single- stranded RNA (ssRNA) associated with protein and RNA polymerase. 

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There are 18 different antigenic subtypes for HA and 11 for NA.

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Variation in Influenza Strains

Antigenic variation in HA and NA is generated by

Antigenic drift involves a series of spontaneous point mutations that occur gradually.

Antigenic shift results in the sudden emergence of a new subtype of influenza.