Host defense abnormalities predisposing the patient to infection.

Considerable progress has recently been made in defining the cellular and molecular mechanisms involved in host resistance to infection. Virtually every decision related to antibiotic therapy is influenced by an assessment of the integrity of these resistance mechanisms. Defects in each major aspect of host defense, that is, humoral immunity, polymorphonuclear leukocyte defense, and cell-mediated immunity, increase the risk of infection caused by specific groups of microorganisms. Knowledge of these defects will guide the initial (empiric) selection of antibiotics, the dosage and duration of antibiotic therapy, and decisions regarding antibiotic prophylaxis. In the severely immunocompromised patient, antimicrobial therapy frequently involves both the administration of antibiotics and the use of treatment modalities that are likely to augment host defenses.