The innate or natural immunity is the first line of defense of the organism and starts working immediately when the presence of a threat is recognized. This includes the skin and epithelial tissues as major parts of the body as well as parts accessible from the outside (such as the respiratory, urinary or digestive tracts). The most important weapon against any threats are phagocytes, which are cells that can 'devour' and destroy the recognized elements as foreign, from simple molecules up to whole cells.

This is the most primitive form of our defense system and, according to the most recent discoveries, it has a fundamental role to activate the mechanism of immunity.