A review of the actions of the host defense against infectious invasion is an important prelude to better understand the ways of escape, and consequently its resistance to some pathogens have adapted.Innate and adaptive response versus host disease. Immunocompetence and susceptibility to infection. Immune mechanisms. Passive and active immunity.First line of defense: prevention and phagocytic cell response. Nonspecific defenses. Defenses skin. Defenses mucosal surfaces. Phagocytosis. NK cells. Lymphatic system. Complement. Chemokines and cytokines. Septic shock.Second line of defense: antibodies and cytotoxic T cells. Activation. Mucosal immunity. Developing specific response. Autoimmune diseases.