Symposium on infections in the compromised host. External barriers to infection.

Knowledge of the external barriers to infection is the first step in understanding the normal defense mechanisms of the host. The intact skin and mucous membranes, with their normal flora and secretions, the action of cilia in the upper respiratory tract, secretory functions, and the normal flow of body fluids are all important factors associated with the external defense against infections. When these barriers are altered or breached, the potential for infection increases and the internal immune system is activated to assist in defending the body.