Microscopically, Mycobacterium avium-complex (MAC) infection is marked by numerous acid fast organisms growing within macrophages. Lots of bright red rods are seen, particularly in macrophages, in this acid fast stain of lymph node. The mycobacterial organisms are naturally resistant to phagocytic destruction, and an immunocompromised state diminishes the TH1 cell mediated immune response that would increase the granulomatous response with macrophage killing of the organisms.

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