Peripheral lymphocytes from patients with chronic lymphocytic leukemia were exposed to phytohemagglutinin in culture. Quantitative assessment of the proliferative response provided an in vitro test of lymphocyte function. The results were correlated with several clinical parameters, both hematologic and immune. Lymphocytes from patients with modestly elevated lymphocyte counts and mild disease manifested slightly delayed and depressed reactions to PHA. All immunoglobulin levels in these patients were also moderately reduced. Further delay and depression of the PHA response showed a rough correlation with progressive lymphocytosis and reduction in circulating immunoglobulins.

Kinetic and autoradiographic analyses have suggested that late-reacting lymphocytes formed distinct populations of abnormal cells which did not seem to be appreciably contaminated with normal lymphocytes, even in instances of mild disease. Only in a patient with splenic lymphosarcoma were multiple circulating populations of normal and late reacting lymphocytes detected simultaneously. In that case, at least some of the late-reacting lymphocytes appeared to originate in parenchymal lymphoid masses.

A definition of CLL could well include, not only the findings in the blood and bone marrow, but a statement of the reaction of the lymphocytes to phytohemagglutinin and of the immunoglobulin characteristics.

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