6.11—Natural, artificial, active and passive immunity

Active immunity follows exposure to an antigen and creates memory cells. It can be natural after infection or artificial after vaccination. Passive immunity transfers antibodies from another source and does not create memory.

Active responses are slower initially but usually longer-lasting; passive protection is immediate but temporary. Vaccination presents antigen without needing the disease itself to establish memory.

Antibodies crossing the placenta are natural passive immunity, while a vaccine is artificial active immunity. An antitoxin injection gives passive antibodies and may need repeating.

Antibody presence does not prove active immunity. A vaccine may need updating when pathogen antigens change, and passive antibodies do not train a secondary response.