All IHC stains will include a positive control tissue

• For the diagnosis of nasal and extra-nasal CD-56 positive lymphomas (these have a characteristic clinical picture, sites of involvement, immunophenotype CD2+, CD3-, CD56+, negative TCR, and Ig gene rearrangements, worse prognosis and a very aggressive behavior. Also, they are ·frequently EBV positive) ^{4, 5}
• Ab can be used to support a diagnosis or to detect minimal residual disease in these cases of CD-56 and NK cell lymphoma
• CD-56 also positive in some other T-cell leukemias/lymphomas. e.g.:
  • Hepatosplenic gamma-delta T-cell lymphoma (usually CD4-, CD8-, CD3+, and consistently show TCR-delta gene rearrangements)
  • S-100 and T-cell lymphoproliferative disorder (CD4-, CD8+, S-100+, CD16+, CD56+, TCR? /?, GR+, EBV-)
• In general B cell lymphomas lack CD56 expressions. ¹
• CD-56 is the most commonly expressed among the available natural killer cell markers
• <0.1% of all lymphocytes in normal or reactive lymph nodes, nasal/nasopharyngeal mucosa, tonsils, thymus, and appendix show positive staining. These positive cells occur singly or at most in tiny groups of 2-3 cells (unlike neoplastic cells)
• Small fraction of plasma cells (1-5%) show weak to moderate cytoplasmic staining
• CD-56 expression is seen in other hemato-lymphoid malignancies and is frequently associated with a worse prognosis (e.g. ALL, AML, multiple myeloma, CD3+, T-cell LGL and lymphoblastic lymphoma) ^{1, 5}
• Small cell carcinomas and carcinoids of the lung are strongly positive in all cells
• Some rhabdomyosarcomas, malignant melanomas and Wilms tumors are also positive²
• Several cases of adenoid cystic carcinomas of bronchial glands are strongly positive
• Some medullary thyroid carcinomas and ovarian tumors are positive⁸
• Nerve fibers are strongly positive and serve as good internal controls
• Other cell types, including sub-populations of breast, prostate, testicular epithelium and ovarian and prostatic interstitial cells have also been found to react in formalin fixed tissue^7,8