(1) The response of a muscle or other living tissue or organism to a stimulus. (2) The color change effected in litmus and certain other organic pigments by contact with substances such as acids or alkalies; also the property that such substances possess to produce this change. (3) In chemistry, the intermolecular action of two or more substances on each other, whereby these substances are caused to disappear, with new ones being formed in their place (chemical reaction). (4) In immunology, in vivo or in vitro action of an antibody on a specific antigen, with or without the involvement of a complement or other components of the immune system. [L. re-, again, backward, + actio, action]