granuloma

A granuloma is the immune system's way of walling off something it cannot easily destroy. When particular cells called macrophages meet a stubborn target, they gather, transform, and form a tight, organized ball of immune cells. Think of it as a microscopic containment vessel, built to quarantine a threat the body cannot simply clear away.

Under the microscope, granulomas are made of activated macrophages, often fused into multinucleated giant cells, surrounded by a rim of lymphocytes. Pathologists distinguish two broad types: caseating granulomas, with a cheesy, dying centre typical of tuberculosis, and non-caseating granulomas, which lack that centre and are characteristic of sarcoidosis.

In the lungs, granulomas appear in a range of conditions. Infections such as tuberculosis and certain fungi provoke them; so do non-infectious diseases like sarcoidosis and hypersensitivity pneumonitis. Because the same structure has many causes, finding granulomas in a biopsy narrows the possibilities but does not by itself give a final answer.

For that reason, granulomas are interpreted together with their appearance, distribution, and special stains or cultures for organisms. A central task is separating infectious granulomatous disease, which needs antimicrobial treatment, from non-infectious causes, where suppressing the immune response may instead be appropriate.