Immunoreactive cytokeratins in plasmacytomas

Fourteen plasma cell tumours, including examples of solitary plasmacytoma and multiple myeloma, were studied with a panel of antibodies reactive in formalin-fixed, paraffin wax-embedded tissue. Each case showed immunoglobulin light chain restriction. Five tumours were reactive with antibodies to cytokeratin. Of these five cases, four were negative with antibodies to leucocyte common antigen and only one was weakly positive. Anti-cytokeratin reactivity by plasma cell tumours is more common than was originally anticipated and represents an important diagnostic pitfall.