Body defense is protection against injury; immunity is protection against foreign micro­organisms or the harmful effect of antigenic sub­stances (antigens).

Antigens are molecules that can stimulate an immune response directed at that specific molecule, and in most cases anti­gens are either components of foreign cells or secretions by microorganisms.

Foreign refers to cells or substances that are not self. Self refers to cells and substances that are normal components of an animal’s body and that normally do not elicit an immune response. The ability to differentiate between foreign and self is a critical function of the immune system. Autoimmune disorders occur when the immune system erroneously identi­fies self tissues or antigens as foreign and mounts an inappropriate immune response. The immune system must also be able to iden­tify and eliminate self cells that have changed so that they may be harmful. The identification and elimination of these altered cells is protec­tion against the development of cancers.

The immune system may be defined as all of the structures and cells involved in providing immune protection. This is not an anatomically

defined system, as cells of this system can be found throughout the body in many tissues. Lymphocytes are the primary cell type involved in an immune response, and the wide distri­bution of lymphocytes throughout the body provides them ready access to invading micro­organisms and newly introduced antigens. Lymphocytes are not a uniform group of cells, and the different subtypes of lymphocytes have specific roles in the overall generation and reg­ulation of an immune response. However, in general, the primary responses are production of circulating antibodies (humoral response), generation of lymphocytes capable of removing the potentially harmful cell (cellular response), or both.

Inflammation can be defined as the response of tissues to injury. The classic signs of acute inflammation (swelling, pain, heat, and redness) are produced as a result of the injury and the tissue response. Chronic inflammatory responses may not demonstrate these classic signs. The desired outcome of the response to injury is complete repair with the restoration of tissues to their original state. obviously, this may or may not be possible, depending on the severity and type of injury and the ability of tissues to respond. While the effects of injury produce local changes that initiate an inflam­matory response, cells attracted to the area of injury (e.g., leukocytes and macrophages) also participate in inflammatory responses.