Peripheral blood lymphocytes from 14 patients with idiopathic aplastic anemia were examined to determine their capacity to differentiate into immunoglobulin (lg) producing cells in the presence of pokeweed mitogen (PWM). Four patients had hypogammaglobulinemia. Lymphocytes from ten patients without and one with hypogammaglobulinemia were capable of differentiating into lg-producing cells. Lymphocytes from three patients with hypogammaglobulinemia, however, were unable to differentiate. Coculture experiments showed that in one patient helper activity of T cells was impaired, in another patient B cells could not be stimulated by PWM to differentiate even in the presence of normal T cells, and in a third patient both the helper function of T cells and the capacity of B cells to differentiate into lg-producing cells were affected. These results suggest that differentiation of lymphoid cells is impaired in some cases of idiopathic aplastic anemia associated with hypogammaglobulinemia.

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