colon; For histological evaluation of a lymph node specimen, it is essential to understand the morphology of the reaction patterns in the normal lymph node after challenge with antigen. The four different immunological reaction patterns seen in the lymph node each take place in their own compartment. Thus the follicle (or germinal) center cell reaction takes place in the follicle, the plasma cell reaction takes place in the medullary cords, the specific cellular response takes place in the paracortex, while in the sinuses, a histiocytic reaction may be observed. Often one reaction is followed or accompanied by another. Furthermore, because different antigenic challenges stimulate the four reactions in different ways, lymph node histology is highly variable and dependent on the kind of antigenic stimulation. Each of the four different reactions (and thus each of the four lymph node compartments) is described morphologically and immunohistologically. Markers identify the B-cell (follicle, medullary cords), Tcell (paracortex), or histiocytic (sinuses) nature of the compartments. A number of malignant conditions that can resemble benign lymph node lesions are discussed briefly, and morphologic and marker criteria for differential diagnosis are reviewed. Recognition of the four different reaction patterns with the addition of marker studies should allow a high percentage of accurate diagnoses
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