THE IMMUNE SYSTEM provides protection against infectious diseases that are caused by various microorganisms including viruses, bacteria, genetic, pathogenic fungi and parasites. The immune systemuses a variety of mechanisms to achieve this goal, including inactivation of biological agents, lysis (rupture) of foreign cells, agglutination (clumping) or precipitation of molecules or cells and phagocytosis (engulfing and inactivating) of foreign agents. Most of the selection emphasis has been on the improvement of growth performance, and these changes have been shown to be negatively associated with immunological parameters of animal that are normally exposed to different kinds of stresses and diseases. Unchallenged birds are optimal for immune competence and disease resistance.