A device (1) for activating light-induced rupture of endocytic vesicles in target cells of a patient so as to effect delivery of an administered antigen to cytosol in the target cells, is described. The device is adapted to be worn by a patient over a region of skin where an antigen and a photosensitising agent have been or are to be administered. The device comprises a rear surface (11) that is rounded or otherwise configured to be worn against the patient's skin. It has a retaining part for retaining the device in place over the region of the patient's skin during an activation cycle, which may be a strap (3a, 3b) or an adhesive layer. A light source, such as an LED, is arranged to illuminate the patient's skin from the rear of the device. A control system is configured to vary the output of the light source with respect to time in accordance with a pre-configured output sequence, which includes an initial stage where the output of the light source is set to be zero or generally below that which could deliver a light dose that can activate light-induced rupture of endocytic vesicles (so as to allow time for the antigen and photosensitising agent to reach the target cells), and a later stage where the output is set to deliver a light dose which can activate light-induced rupture of the endocytic vesicles (for effecting the delivery of the administered antigen to the cytosol of the target cells).
