After infection with bacillus Calmette-Guérin, peritoneal macrophages (Mø) display enhanced expression of FcR for both monomeric and complexed IgG2a, but not IgG2b. Isotype specificity of FcR can be reversed on nonactivated Mø by immune lymphokines, and IgG2a immune complexes are more effective triggers of the respiratory burst in activated Mø. Selective enhancement of IgG2a FcR by Mø activation could account for efficacy of homologous ab in mediating cytotoxicity in some systems.

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