In contrast to innate immunity. The

In contrast to innate immunity. There are other immune responses that stimulated by exposure to infectious agents and increase in magnitude and defensive capabilities with each successive exposure to a particular microbe. Because this form of immunity develops as a response to infection and adapts to the infection. It is called adaptive immunity. The defining characteristics of adaptive immunity are exquisite specificity for distinct molecules and an ability to "remember" and respond more vigorously to repeated exposures to the same microbe. The adaptive immune system is able to recognize and react to a large number of microbial and nonmicrobial substances. In addition, it has an extraordinary capacity to distinguish between different, even closely related, microbes and molecules, and for this reason it is also called specific immunity. It is also sometimes called acquired immunity to emphasize that potent protective responses are "acquired" by experience. The main components of adaptive immunity are cells called Iymphocytes（淋巴细胞） and their secreted products, such as antibodies. Foreigh substances that induce specific immune responses or are recognized by Iymphocytes or antibodies are called antigens.