Natural host-resistance mechanisms are essential first-line defenses against most mycotic agents; however, these defenses are often not sufficient for complete protection. The host relies on the immune responses to provide the additional antifungal activity necessary for maximum protection. In systemic mycotic diseases, the immune system must be functional to prevent the host from succumbing to the disease. Natural resistance mechanisms act together in a coordinated manner early in the disease process to either kill or prevent the fungal agent from proliferating and gaining entrance to other tissues. The early defensive measures of the natural effector mechanisms are usually sufficiently effective to provide time for the generally more effective immune defenses to develop. Together, the natural defenses and the immune defenses regulate each other through cytokine networks, and generally these systems provide the host with adequate protection against threatening mycotic agents.