cartridge extractor: a mechanism in a firearm that pulls an empty shell case out of the chamber and passes it to the ejector
mechanism: device consisting of a piece of machinery; has moving parts that perform some function
acquired immunity: immunity to a particular disease that is not innate but has been acquired during life; immunity can be acquired by the development of antibodies after an attack of an infectious disease or by a pregnant mother passing antibodies through the placenta to a fetus or by vaccination