CD8 is a glycoprotein, 30-34 kDa, a MHC class I restricted receptor. CD8 binds to the nonpolymorphic region of class I molecules; it may increase avidity of cell-cell interactions. The epitope defines the CD8 suppressor-cytotoxic subtype of T-cells. CD8 positive T-cells are mainly involved in cytotoxic immunoreactions. The CD8 antigen is found on 80% of the thymocytes. In corticaL thymocytes, CD4 and CD8 are co-expressed. About 20-35% of peripheral blood lymphocytes are CD8 positive. The CD8 antigen is also found on NK cells and splenic littoral cells. 80% of the intraepithelial T-cells are CD8 positive. In refractory celiac disease, CD8 positivity is lost from the intraepithelial T-cells.
CD8 is detected in mature T-cell lymphomas, generally less often than CD4. T-cell large granular lymphocyte leukaemia and subcutaneous panniculitis-like T-cell lymphoma are most often CD8 positive (while CD4 negative).
Classification of lymphocytes, e.g., in inflammatory lesions. Classification of malignant lymphomas (see above).
Tonsil and appendix are recommended as positive and negative tissue controls for CD8. In tonsil, the protocol must be calibrated to provide a distinct and strong membranous staining reaction of virtually all suppressor/cytotoxic T-cells. In appendix, dispersed suppressor/cytotoxic T-cells primarily located in lamina propria mucosa but also situated in the appendiceal epithelium must be strongly stained, whereas the epithelial cells should be negative.
No staining reaction must be seen in other cells including B-cells, stromal cells or squamous epithelial cells of the tonsil.
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