Test Details: Immunohistochemical testing helps to identify specific antigens (proteins) within tissue sections utilizing an antigen-specific antibody. PAX-5 (also known as B-cell activator protein) is a B-cell-specific transcription factor and expressed in the early pro-B, pre-B, and naive stages of B-cell development until the mature B cells. It is expressed in B-cell lymphoma/leukemia, Reed-Sternberg cells of Classic Hodgkin lymphoma.