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926 THE EFFECT OF PHENYTOIN ON IMMUNOGLOBULIN (Ig) PRODUCTION

Patients treated for epilepsy with phenytoin develop a significant fall in serum IgA. We investigated this association because it may be a model for selective IgA deficiency. Phenytoin (20μg/ml) was added to human lymphocyte cultures stimulated by pokeweed mitogen (PWM). After 6 days in culture lymphocyte proliferation was measured by thymidine incorporation. B-cell differentiation and relative proportions of IgG-, IgM- and IgA- containing plasma cells (PC) were measured by counting PC in cytocentrifuge preparations stained for Ig.These results suggest that phenytoin at therapeutic levels does not interfere with PWM-stimulated thymidine incorporation. B-cell differentiation to PC was reduced but isotype distribution was unchanged. This latter finding is consistant with the published observation that IgG and IgM, as well as IgA, are lowered in some patients on phenytoin. The predominance of the in vivo effect on IgA may be due to a genetic predisposition, interference with T-cell cooperation needed for B-cell production of IgA, or direct interference by the phenytoin molecule in the biochemical basis of B-cell differentiation and proliferation.