Pathogenesis of post primary tuberculosis: immunity and hypersensitivity in the development of cavities.

M. Tuberculosis (MTB) is an obligate human parasite even though humans are more resistant than any of the animals used for study. It is a human parasite because only humans develop post primary tuberculosis (TB) in their lungs that mediates transmission of infection to new hosts. The extreme paucity of human lung tissue with post primary TB has forced scientists to study animal models and human tissues that do not have the disease. Consequently, the unique features of post primary TB remain largely unknown and misconceptions are widely accepted. This manuscript presents a revised pathogenesis of post primary TB based on studies of lung tissues of thousands of patients by multiple authors and related literature. Primary TB stimulates systemic immunity that kills organisms and heals granulomas resulting in both protection from disseminated TB and resistance to new infection. Post primary TB, in contrast, requires systemic immunity that it subverts to produce local susceptibility in the apex of the lung. It begins in the part of lung with the lowest ventilation, perfusion and movement and then proceeds to paralyze alveolar macrophages, block the exits and suppress inflammation to further isolate the area with post obstructive pneumonia. This provides a safe place for a small number of MTB to drive prolonged accumulation of host lipids and mycobacterial antigens in an otherwise immune person. After many months, the affected lung suddenly undergoes caseation necrosis with vanishingly few MTB. The necrotic tissue fragments to produce a cavity or hardens to develop fibrocaseous disease. Evidence suggests that this is triggered by a hypersensitivity reaction against cord factor and then progresses as the Koch phenomenon against many antigens. MTB grow in perfusion only in dead tissue or on a cavity wall. We anticipate that a more accurate understanding of the pathogenesis of post primary TB will facilitate focusing modern technologies to produce rapid advances in understanding and combating TB.