Microorganisms and Disease
Type III secretion systems are used to inject "effector" proteins directly into a host cell. Salmonella uses a type III secretion system to help the pathogen to

survive the acid pH of the stomach
activate T cells to proliferate and secrete cytokines
secrete LT (heat labile toxin) and ST (heat stable toxin) into the lumen of the intestine
survive within macrophages

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Microorganisms and Disease
The influenza vaccine is administered each year because

None of these
the vaccine is sufficiently toxic to make it necessary to administer only a small amount at any one time
mutations in the viral hemagglutinin may allow the virus to evade the immune response elicited by previous vaccines
it is a polysaccharide vaccine that does not confer long-term protection

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Microorganisms and Disease
Cholera toxin is an A-B type toxin in which the B portions bind to a receptor on a host cell and the A portion enters the cell to cause

ADP ribosylation of a G protein that locks it into an active form that stimulates adenylate cyclase to make cAMP
ADP ribosylation of adenylate cyclase that stops production of cAMP
ADP ribosylation of guanylate cyclase that stimulates production of cGMP
cleavage of rRNA that results in disruption of ribosome function

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