Immune System

The immune systemcomprises an interacting assemblageof cells and solublemolecules whose primary function is to kill invading microorganisms that may cause damage to the body. Two interdependent kinds of immune systemare present inmost vertebrates, which together trigger one or more different killing mechanisms according to whether themicrobes live within or outside the cells of the body. An innate system,mediated by receptors which recognize uniquelymicrobial structures, responds rapidly to the threat of invading organisms. This underlies an adaptive system, mediated by antigen receptors on lymphocytes, which produces a more sustained and comprehensive response. Only the adaptive system, found exclusively in vertebrates, retains amemory of exposure to each microbe and ensures the system is mobilized more rapidly upon a subsequent infection by the same pathogen.