Serial IgG determinations were carried out on 29 children with acute leukemia. With the exception of the pretreatment assays, all determinations were performed during the course of specific antileukemic therapy. Following the initial introduction to cytotoxic agents, a significant, although temporary, reduction in IgG level occurred. The continuous use of antileukemic agents did not alter the IgG levels. Relapses, responsive to changes in therapy, were usually associated with moderate reductions in the IgG levels. Greater reductions in IgG levels occurred following the institution of changes in therapy. In both instances the declines were followed by a return in IgG to the normal range. Declines of significantly greater magnitude and rate occurred in those patients no longer capable of responding to therapy changes. In such patients, levels below 3.8 to 3.5 mg./ml. signified impending death, a lack of response to infection and inability to evoke an IgG response. An example of the interrelationships between clinical and immunologic responsiveness is shown by the patient in complete relapse with a normal IgG assay, who went into remission and had an impressive IgG response following a rubeola infection. Shortly thereafter, her disease process entered an irreversible relapse stage with an IgG level of 2.9 mg./ml., and death occurred within 30 days following a bacterial septicemia.

The relationship between hematologic and immunologic responsiveness raises the provocative question of paradoxically maintaining an immunologically responsive state as a means of prolonging the survival time in the face of persistent immunosuppressive therapy.

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