Résumé : BACKGROUND: Repeated administration of chimeric or humanized monoclonal antibodies is generally well tolerated. Anti-idiotypic sensitization is rare and is considered to be of no clinical significance. We observed a child who experienced anaphylactic shock when he received a second course of basiliximab at the time of his second renal transplantation. We therefore searched for the presence of anti-basiliximab immunoglobulin (Ig) E in this patient. METHODS: Serum levels of anti-basiliximab IgE, assay of the anti-murine reactivity of circulating anti-basiliximab IgE, and assays for serum anti-mouse antibodies and global anti-basiliximab anti-idiotypic antibodies were carried out by enzyme-linked immunosorbent assay. Anti-basiliximab IgE antibodies on circulating basophils were evaluated by the ability of the patient's blood to produce leukotrienes in vitro after exposure to basiliximab. RESULTS: Sequential assays of serum samples by enzyme-linked immunosorbent assay indicated that anti-basiliximab IgE antibodies appeared after the second basiliximab course. There was no IgE reactivity toward a control murine IgG2a monoclonal antibody (mAb), indicating that the IgE response was directed exclusively against basiliximab idiotypes. There was no IgE reactivity against the humanized anti-interleukin-2 receptor mAb daclizumab, which was derived from a distinct parental murine mAb. Patient basophils harvested months after the anaphylactic shock produced leukotrienes in vitro on exposure to basiliximab. CONCLUSIONS: Patients exposed to chimeric antibodies may develop an anti-idiotypic IgE response that can trigger anaphylactic shock on further exposure. Specific anti-idiotypic IgE may be bound to basophils even in the absence of circulating IgE.