Lymphadenitides are acute and chronic inflammatory processes of lymph nodes that occur in response to a variety of pathogenic agents. They may be specific or nonspecific, featuring necrosis, abscesses, granulomas, and fibrosis in various combinations. Microorganisms causing lymphadenitides are viruses, bacteria, fungi, and protozoa. The cellular reactions in each category are characteristic and contributory to the histologic diagnosis. Adjuvant diagnostic methods such as special stains, electron microscopy, immunohistochemistry, and in situ hybridization analysis, and polymerase chain reaction (PCR) techniques can be used in the identification of the etiologic microorganisms.