Immunostaining of small-cell carcinoma: mistaken identity?

Sir-We can say with certainty that the cells in question were small-cell carcinoma cells and not red blood cells. First, the renal glomeruli in this case contained only a few red cells, as shown by standard haematoxylin and eosin stains, but did contain many nucleated cells similar to those in the primary lung cancer. Second, the appropriate blocking procedures were used to prepare the immunoperoxidase preparations. Third, the cytoplasm of the cells in the glomeruli stained positive for cytokeratin, which is not a component of red cells and is not found in the tumour-cell nuclei. Fourth, the cells in the glomeruli also stained positive for TTF-1 when alkaline phosphatase was used in place of peroxidase. Fifth, staining for haemoglobin was negative.