Final answer:
Cavitary lesions are formed as a result of bacteria surviving inside macrophages and the macrophage-driven immune response that causes chronic inflammation, tissue damage, and necrosis, leading to the formation of these lesions.
Step-by-step explanation:
Cavitary lesions can be formed through the activity of both bacteria and the immune response by macrophages. When bacteria such as Mycobacterium tuberculosis are inhaled into the alveoli, they are phagocytized by macrophages. However, due to the protective waxy mycolic acid in the bacterial cell walls, the bacteria can survive and multiply within these phagocytes. If the infection is not eradicated by macrophages, it leads to an inflammatory response, where neutrophils and macrophages accumulate, and ultimately to the formation of tubercles. Over time, the immune response causes tissue damage and apoptosis, leading to liquefaction and the creation of an air pocket, where aerobic bacteria can thrive, known as a cavitary lesion.
The process is similar in other diseases like histoplasmosis, where the fungal spores result in granulomatous lesions. Infected macrophages, upon receiving signals from Th1 cells, try to destroy the intracellular bacteria with oxidative metabolism and nitric oxide production. In cases where macrophages can't eliminate pathogens, the continued immune response contributes to the development of lesions.
In summary, cavitary lesions are created by the combination of bacterial survival tactics and the immune system's response to infection, particularly through the action of macrophages and the chronic immune response leading to tissue damage and necrosis.