Question

if a person who was vaccinated against measles as a child is exposed to the measles virus as an adult, they typically do not get sick. which of the following best explains why? the measles virus is no longer harmful to adults, only to children. the original antibodies from the childhood vaccination are still circulating in their blood decades later. the adults innate immune system is strong enough to fight off any pathogen without prior exposure. memory cells, formed after the vaccination, launch a rapid and effective immune response against the virus.

Explanation:

Vaccination stimulates the immune - system to produce memory cells. When exposed to the pathogen later, these memory cells quickly recognize and mount an immune response, preventing illness. Antibodies from childhood vaccination don't typically circulate for decades. The measles virus can still be harmful to adults, and the innate immune system isn't capable of fighting off any pathogen without prior exposure.

Answer:

Memory cells, formed after the vaccination, launch a rapid and effective immune response against the virus.