Whole blood assays are very informative as they contain elements i.e. autologous plasma, platelets, red blood cells usually missing from in vitro experiments based on the use of cell lines grown in fetal calf serum (FCS) or PBMC isolated from a buffy coat. With carefully designed whole blood assays you can detect contaminants, assess efficacy, predict species cross reactivity and validate results on multiple donors.
By using whole blood assays, you can confirm your in vitro data before starting expensive in vivo/ex vivo assays or considering the use of sophisticated and expensive models (e.g. human stem cells).