CD4 is a lymphocyte surface marker that is found on T helper lymphocytes. It is also expressed by Langerhans cells and other macrophages. In the peripheral circulation, there is normally a 4:1 ratio of CD4+ helper cells to CD8+ cytotoxic/suppressor lymphocytes. The combined use of CD4 and CD8 enables the diagnostic dermatopathologist to assess for an imbalance in this ratio. Most T cell lymphomas will demonstrate an alteration in the ratio.
Newer antibodies directed against CD4 reliably label T helper cells in formalin-fixed, paraffin-embedded tissue sections. When used in conjunction with antibodies directed against CD8, estimations of CD4/CD8 can be determined, helping in establishing a diagnosis of mycosis fungoides (Fig. 9) (25,26). In B cell lymphomas, an admixture of CD4 and CD8 positive reactive T lymphocytes are present.
Anti-CD4 antibodies that survive formalin-fixation and paraffin embedding recently have become commercially available. The antibodies work very well in routinely processed tissue and do not require enzymatic pretreatment. In our laboratory, we have attained superior staining results with HIER pretreatment using a citrate buffer.
Antibodies directed against CD4 epitopes play an essential role in the subtyping of cutaneous lymphomas. They are helpful in establishing or refuting a diagnosis of mycosis fungoides. Their use, in conjunction with anti-CD8 antibodies, is the ideal way to characterize the immunologic phenotype of dermal lymphocytic infiltrates.
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