The majority of monoclonal antibodies having potential for in vivo diagnostic and therapeutic applications, in man, are of mouse or rat origin. They need to be "humanized" to prevent rejection by the human immune system and, where indicated, to activate host effector mechanisms. Genetic engineering techniques have made all nine human immunoglobulin isotypes available, but we do not have a sufficient understanding of their functional activities to determine which isotype may be optimal for a given application. This review discusses the major effector systems identified and recent studies directed at defining the structural basis for immunoglobulin recognition by Fc receptors and complement. Site-directed mutagenesis has demonstrated that ligand-binding sites can be modulated by single amino acid substitutions. Additionally, it has been shown that glycosylation is essential for full functional activity. It appears that there is a subtle interplay between the polypeptide and oligosaccharide structures to generate these ligand binding sites. Taken together, these findings suggest that it will be possible to generate "customized" antibody molecules having a pre-determined profile of effector functions.