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

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Microorganisms and Disease
A major difference between EHEC and EPEC is

EHEC possesses a type III secretion system and EPEC does not
EHEC secretes a Shiga-like toxin and EPEC does not
EPEC rearranges host cell actin and EHEC does not
EPEC passes through the placenta to infect the fetus and EHEC does not

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Microorganisms and Disease
The influenza vaccine is administered each year because

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

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