Hairy cell leukemia: a B cell neoplasm with a unique antigenic phenotype.

A panel of B, T, and monocyte antibodies was used to study the phenotype of hairy cell leukemia. It was found that the neoplastic cells had monoclonal surface immunoglobulins and, in addition, reacted with B1, HLA-DR, Leu 10, and Tac. Most of the above markers are features of B cell lymphocytes. However, Tac is a monoclonal antibody normally reactive with activated T lymphocytes. All other antibodies reactive with T cells were negative. An additional unexpected finding was intranuclear reactivity in 4/5 cases with an antibody to TdT. TdT enzymatic activity was not demonstrable by biochemical assay. All five cases were nonreactive with monocyte antibodies. The findings support the concept that hairy cell leukemia is of B cell lineage but demonstrates an unusual phenotype not seen in other normal or neoplastic B lymphocytes.