Ghon complex

In primary tuberculosis inhaled bacilli reach the well-ventilated lower part of the upper lobe or upper part of the lower lobe, where a caseating granuloma forms — the Ghon focus — and organisms drain to hilar and mediastinal nodes, which enlarge and caseate. Together these constitute the Ghon complex, and the combination of a small peripheral lesion with disproportionately large hilar nodes on chest radiograph is characteristic of primary disease in a child. Most primary infections are contained by cell-mediated immunity within weeks; the lesion heals by fibrosis and calcification into a Ranke complex, and viable bacilli may persist in a dormant state, producing latent infection detectable by tuberculin skin test or interferon-gamma release assay. Reactivation typically occurs in the apical and posterior segments of the upper lobes, where oxygen tension is highest, and produces cavitary disease. Progressive primary tuberculosis, with lymphohaematogenous dissemination to miliary or meningeal disease, occurs in young children and the immunocompromised.