week 6 - infection and immune response 2 Flashcards
examples of diseases caused by viruses
polio, hepatitis B, rotavirus, measles, mumps, rubella, influenza, human papilomavirus
positive-sense viral RNA
similar to mRNA and so can be immediately translated by host cell
negative-sense viral RNA
complementary to mRNA so must be converted to positive-sense RNA before translation
describe a viral genome
can be DNA or RNA
linear or circular
ss or ds
structure of a virus
genome protected by a capsid made from capsomeres = nucleocapsid
capsid can be helical or icosahedral
some viruses have an envelope on outside which is derived from host cell membrane and has virus proteins and glycoproteins inserted
what kind of virus causes hepatitis B
ds circular DNA virus
positive sense ss RNA virus examples
coronavirus poliovirus Zika virus norovirus rubella
ss negative sense RNA virus examples
Ebola virus
mumps virus
influenza virus
measles virus
viral tropism
ability of a virus to productively infect a particular cell, tissue or host
stages of viral infection and replication
attachment - viral proteins interact with specific receptors on host cell
penetration - attachment induces conformational change in viral proteins - results in fusion of viral and cellular membranes
uncoating - viral capsid removed and viral genome is released
replication - viral genomes replicated - process depends on if virus is RNA or DNA
assembly - viral proteins packaged with newly replicated viral genome into visions that are released
vision release - lysis or budding
DNA virus replication
viral DNA enters host cell nucleus (may become incorporated into host DNA)
transcription into mRNA is catalysed by host cells RNA polymerase
mRNA translated into virus-specific proteins
coat proteins assemble around viral DNA and visions are released by budding or lysis
RNA virus replication
dsRNA viruses - one strand is transcribed into mRNA
ssRNA - positive sense is used directly as mRNA - negative sense is transcribed into positive sense which can then be used as mRNA
mRNA is then translated into proteins
retrovirus replication
reverse transcriptase synthesises DNA from RNA which can then be integrated into host genome and is called a provirus
Provirus DNA is transcribed into both new viral genome RNA as well as mRNA for translation in the host into viral proteins
visions released by budding
how does HIV invade cells
HIV has a glycoprotein called gp120 which has a high affinity for CD4
infection of T cells is assisted by T cell co-receptor called CXCR4 - HIV also infects monocytes by interacting with CCR5 co-receptor
outcomes of a viral infection
virus is released and host cell is destroyed by cell lysis
virus can exit by budding
virus can be maintained in host cell cytoplasm
virus can become incorporated into genome
can become an oncogenic virus causing uncontrolled cell growth
three main mechanisms for antigenic variation
mutations
recombination - DNA strands break and covalently link the DNA fragments, either from a single gene or from two infecting viruses of the same kind
gene reassortment - viruses can exchange genetic material - extends gene pool
antigenic drift
small antigenic changes that are sufficient to reduce the effectiveness of b and t cell memory eg. Influenza virus and HIV
leads to new strains that go unrecognised by host
does not change the viral subtype
antigenic shift
exchange of genetic material between two pathogens making a new hybrid virus
sterilising immunity
innate and adaptive response
results in recovery
non-sterilising immunity
has a good immune response but fails to clear infection completely resulting in chronic infection
causative organism of tuberculosis
mycobacterium
pathology of TB
apex of lung is predominantly affected - normal spongy architecture replaced by caseous
In the middle of the granuloma is caseous necrosis - surrounding it is a layer of highly active macrophages – surrounding the macrophages are sheets of lymphocytes (predominantly t cells) – scattered around a typical TB granuloma are giant cells
triggers for reactivation of MTB
Immunosuppression (particularly a co-infection of HIV)
Age – reactivation later in life