Host determinants of susceptibility to Mycobacterium tuberculosis.

Susceptibility to tuberculosis is determined by a number of host factors and may vary according to characteristics of the infecting strain. Native resistance is probably mediated via the macrophage and is under genetic control. Other factors that contribute to native resistance are nutrition, associated disease states, age, and sex. Acquired resistance is mediated via macrophages and controlled by T-helper cells. The ability of an activated macrophage to phagocytize and kill virulent tubercle bacilli is influenced by the function and number of T-helper cells. The macrophages are affected by concurrent disease states and immunosuppressive drugs. Macrophages provide local immunity, once activated, and T-helper lymphocytes provide the long-term systemic immunity.