Methods are provided for treating blood monocytes to produce functional antigen presenting dendritic cells. An extracorporeal quantity of a subject's blood is treated to separate the blood into a plasma component containing proteins, a platelet component and a buffy coat component. A plastic treatment device is provided having plastic channels that allow transmittance of light to the interior of the plastic device and a light source that produces light of a wave length selected to activate the photoactivatable agent. The plasma component containing proteins is first pumped through the plastic treatment device, followed by the platelet component and finally the buffy coat component. The resulting treated cells may be incubated or reinfused directly to the subject.