Note: Tolerance to specific foreign antigens can be induced by the exposure to the foreign antigens during embryonic or neonatal life (depending upon species). In adults, tolerance (usually of shorter duration) can be induced by using particular routes of administration for the antigens or administration of agents that are particularly effective against cells proliferating in response to antigen. Mechanisms may include deletion of potentially reactive lymphocytes or their "inactivation" by immunological suppression.