The protein recognized by B28p is expressed in mature B cells and plasma cells. (A) In normal tonsil, B28p reacts with B cells and, more strongly, with plasma cells (arrow); notice the weak positivity of tonsil epithelium (E). (B) The expression pattern of IRTA1 in normal tonsil (as revealed by the M-IRTA1 mAb) clearly differs from that of TPD52, labeling being restricted to a subset of B cells located within and beneath the tonsil epithelium (E). GC indicates germinal center. (C) The expression pattern of TPD52 in a reactive lymph node is similar to that observed in the tonsil; the arrow points to an aggregate of strongly positive plasma cells. (D) In a reactive bone marrow, TPD52 is strongly expressed in plasma cells (arrow) and B cells (arrowhead), while hematopoietic precursors are negative. The double arrow points to a TPD52-negative megakaryocyte. T indicates a bone trabecula. The APAAP technique was used with hematoxylin counterstain. Original magnifications, × 200 (A-C) and × 1000 (D).