The skin is a boundary layer through which organisms can enter only if there is a tear or cut. Similarly, the mucous linings in the nose, mouth and along the trachea prevent invaders from entering from the inside. Stomach acids, peristaltic contractions in the intestine and rapid shedding of intestinal linings can destroy organisms that get that far. If an invader should get past these initial defenses, the body's first response to injury is inflammation. During the inflammatory process, the blood supply to the injured area increases.
Several systems interact to limit damage to surrounding tissue.
Other cells called natural killer cells can attach to cells that have been infected and destroy them. The primary actors at this level are the lymphocytes, a type of white blood cell. T-cells are the major actors in cellular immunity. Cellular immunity is responsible for fighting off viruses and certain bacterial infections and eliminating cancer cells. T-cells have immunoglobulins attached that recognize specific foreign invaders.