Isotype controls are conjugated antibodies that lack specificity to the target which can be used as negative controls to estimate the level of staining background and non-specific binding in an experiment. Indeed, conjugated antibodies can react non-specifically with cells in different ways (e.g. binding to the receptor of cells expressing Fc receptors).
Beckman Coulter provides a wide range of isotype control covering most of the isotype species and classes, and fluorochromes commercialized. Isotype controls have the same F:P ratio as conjugated antibodies of their respective product line.