"action in resistance or response to another action or power," 1640s, from re- "back, again, anew" + action (q.v.). Modeled on French réaction, older Italian reattione, from Medieval Latin reactionem (nominative reactio), a noun of action formed in Late Latin from the past-participle stem of Latin reagere "react," from re- "back" + agere "to do, perform."
Originally a word in physics and dynamics. In chemistry, "mutual or reciprocal action of chemical agents upon each other," by 1836. The general sense of "action or feeling in response" (to a statement, event, etc.) is recorded from 1914. Reaction time, "time elapsing between the action of an external stimulus and the giving of a signal in reply," attested by 1874.
mid-15c., "free, exempt" (from taxes, tithes, sin, etc.), from Latin immunis "exempt from public service, untaxed; unburdened, not tributary," literally "not paying a share," from assimilated form of in- "not, opposite of" (see in- (1)) + munis "performing services" (compare municipal), from PIE *moi-n-es-, suffixed form of root *mei- (1) "to change, go, move." Specific modern medical sense of "exempt (from a disease)," typically because of inoculation, is from 1881, a back-formation from immunity. Immune system attested by 1917.
also auto-immune, "arising from an abnormal immune response to a normal body part," 1952, from auto- + immune. Related: Autoimmunity, attested by 1903 as "immunity, natural or acquired, effected by the unaided powers of the organism, independent of external agencies." The modern sense of "immune responses of an organism against its own healthy cells and tissues" is from 1950s.
1969, from combining form of immune + deficiency.
c. 1600, "gained by effort," past-participle adjective from acquire. Of diseases, "occurring after birth, thus not dependent on heredity," 1842 (opposed to congenital); acquired immune deficiency is attested by 1980; acquired immune deficiency syndrome by 1982. Acquired taste is attested from 1734.
1831, "of or pertaining to political reaction, tending to revert from a more to a less advanced policy," on model of French réactionnaire (19c.), from réaction (see reaction). In Marxist use by 1858 as "tending toward reversing existing tendencies," opposed to revolutionary and used opprobriously in reference to opponents of communism. Non-political use, "of or pertaining to a (chemical, etc.) reaction" (1847) is rare. As a noun, "person considered reactionary," especially in politics, one who seeks to check or undo political action, by 1855.
1982, acronym formed from acquired immune deficiency syndrome. AIDS cocktail attested by 1997, the thing itself said to have been in use from 1995.
