Mycobacterium tuberculosis has tough outer coat which is poorly antigenic and not easily phagocytosed. Therefore, organism may break out years after first infection - reactivation if host resistance dips. The organism is a facultative aerobe, and thrives when in contact with oxygen in the airways. First encounter is called primary TB , usually asymptomatic. Pulmonary parenchymal lesion is peripheral, in mid zones, i.e., upper part of lower or lower part of upper lobe.
Organism invades as far as hilar nodes before immunologic response is triggered.
In most cases, organism is contained by fibrosis, within macrophage cytoplasm, as calcified primary lesion. Host resistance and antimicrobial therapy may arrest this phase. This response kills host tissue, as well as the organism, hence caseation necrosis in tubercles. In primary TB , the organism usually disseminates to the oxygen-rich lung apices before being contained. Secondary TB often reactivates here.