Immunity is the response of the body to microorganisms and foreign materials. It may be innate, i.e. inborn, or acquired, i.e. developed after birth. Humans are immune to many diseases which affect other animals. This innate immunity is a feature of the species to which humans or other animals belong and does not depend upon any defense mechanism being set up after birth. Acquired immunity is developed during life and may be either active, resulting from antibodies formed by the body, or passive resulting from antibodies donated from another animal. The acquired immune system is able to recognize and react specifically to foreign material (usually called an antigen), but does not normally react to the body’s own component. Two main types of immunological defense reactions can be triggered by antigens entering the body are the humoral and the cell-mediated. It can be seen that the immune response consists of certain cell, proteins and tissues which function together to provide protection from foreign material entering the body. Initially, the immune response appears to play a protective role, preventing plaque microorganism from penetrating the gingival tissues. Abnormalities of the immune response are hypersensitivity and autoimmunity.