In one area of the lung you notice a large lesion with a cheesy interior surrounded by fibrous tissue.

What type of necrosis is this?

What might it be caused by?

Is it an acute or chronic response?

This is a large caseating granuloma. In this case it is caused by the tuberculous bacillus, which can be "acid fast" stained as in the virtual slide. Often only a few bacilli can be detected after an exhaustive high power search. Caseating granulomas form when the immune system is unable to eliminate a pathogen, particularly acid-fast bacilli and fungi. Instead it walls off the surrounding area with a chronic inflammatory capsule that contains lymphocytes, macrophages, multinucleated giant cells, fibroblasts and fibrous tissue. In the center is a mixture of caseous necrotic debris that can contain live pathogen for decades.