There are two outstanding varieties of foreign body granuloma—one is produced by animate and the other by inanimate material. The foreign substance may be introduced from without, as in injuries, or produced in the body, as in necrotic areas where cholesterol or fatty acid crystals are formed. Occasionally the foreign substance may be inserted in the form of sutures or ligatures in surgical operations or fortuitously as dusting powders on surgeons' rubber gloves, or even deliberately for "beautification."
The tubercle is the most prevalent and familiar example of foreign body granuloma due to a living substance. If, however, as Prudden and Hodenpyle showed many years ago, the tubercle bacillus is killed and dead bacilli are insufflated into the lungs of rabbits, the bacilli produce epithelioid tubercles which are histologically typical except, of course, for the absence of caseation. Nor, obviously, can they reproduce similar lesions locally or in distant tissues.