Malakoplakia, a condition often associated with immunodeficiency and characterized by abnormal macrophage function consists of sheets of large histiocytes in response to a bacterial infection. It represents a relatively ineffective inflammatory response because of persistence of bacteria and/or bacterial degradation products within the histiocytes. In the lungs it is almost always caused by infection with Rhodococcus equi but it may be caused by other bacteria. Malakoplakia is morphologically very similar to Mycobacterium avium complex infection in AIDS patients and to Whipples disease and lepromatous leprosy. The distinguishing and diagnostic feature of malakoplakia is the presence of small intracellular concentrically calcified structures called Michaelis-Gutman bodies (MG bodies). MG bodies also contain stainable iron. Intracytoplasmic calcifications not having the typical appearance of MG bodies may also be present.