A tubercle is a small nodular lesion characteristic of granulomatous inflammation, particularly seen in tuberculosis. It consists of a collection of immune cells that wall off an infectious focus.
Structure and Development
Tubercles form when host immune responses attempt to contain pathogens such as Mycobacterium tuberculosis. Inhaled bacilli are phagocytosed by alveolar macrophages; if the bacteria survive, they multiply and attract additional immune cells. Activated macrophages transform into epithelioid cells and may fuse to form Langhans giant cells. Lymphocytes surround the lesion, and fibroblasts deposit a fibrous rim. The center often undergoes caseous necrosis, giving tubercles a cheesy appearance on histology. These granulomas serve to localize infection but also provide a niche for persistence, leading to latent disease. When immunity wanes, the caseous center can liquefy and discharge bacilli into airways, leading to cavitary tuberculosis and transmission. The initial lung lesion, called a Ghon focus, may heal with calcification; involvement of regional lymph nodes forms a Ghon complex.
Forms and Clinical Significance
Tubercles vary in size and distribution. In miliary tuberculosis, numerous tiny tubercles spread through the lungs or systemic organs via the bloodstream. In secondary tuberculosis, large caseating tubercles may coalesce and cavitate. Similar granulomatous lesions can appear in bones, kidneys, meninges or lymph nodes. Avian and bovine tuberculosis produce tubercles in livestock, affecting food safety. Other diseases, such as sarcoidosis or fungal infections, may form non‑caseating tubercles, highlighting the need for microbiological confirmation. Radiographic imaging often shows calcified tubercles, and biopsy reveals the characteristic cellular architecture.
Tubercles represent the host’s attempt to contain persistent pathogens. Understanding how these granulomas form and break down is central to tuberculosis pathogenesis and control.
Related Terms: granuloma, tuberculosis, Mycobacterium tuberculosis, caseous necrosis, Ghon complex