The REA Control (S) antibody clone REA293 is a universal isotype control that can be used with all recombinant engineered antibodies (REAfinity™ Antibodies) that recognize cell surface antigens. REAfinity Antibodies have been engineered for their high specificity and contain human IgG1 parts for constant regions. Although REAfinity Antibodies show virtually no binding to Fc receptors, the use of the clone REA293 is still recommended to control for other non Fc receptor–mediated non-specific binding of fluorochrome-conjugated REAfinity Antibodies to cells. Unspecific interactions of the fluorochrome with the cell surface can also be controlled for with conjugated versions of clone REA293.