A device 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, the device being adapted to be worn by a patient over a region of the patient's skin where the antigen and a photosensitising agent have been or are to be administered, wherein the device comprises: a rear surface that is configured to be worn against the patient's skin; a retaining part (3a, 3b; 3c; 3d; 15) for retaining the device in place over the region of the patient's skin during an activation cycle; a light source arranged to illuminate the region of the patient's skin from the rear surface of the device; a control system to control the operation of the light source after initiation of the activation cycle; and a power supply to power the light source and the control system, characterised in that the control system is configured, after the initiation of an activation cycle, to vary the output of the light source with respect to time in accordance with a pre-configured output sequence, wherein the output sequence includes an initial stage and a later stage wherein a later stage light dose is output; the initial stage duration being of at least six hours and the output of the light source during the initial stage being set to be zero or below the later stage light dose to avoid the light-induced rupture of endocytic vesicles, to allow time for the antigen and photosensitising agent to reach the target cells; and wherein during the later stage the output of the light source is set to deliver the later stage light dose to activate light-induced rupture of the endocytic vesicles for effecting the delivery of the administered antigen to the cytosol of the target cells, wherein the later stage light dose is at least 0.05 J/cm 2 and is delivered over 5 minutes to 15 hours at an intensity of 0.5 mW/cm 2 or less. a rear surface that is configured to be worn against the patient's skin; a retaining part (3a, 3b; 3c; 3d; 15) for retaining the device in place over the region of the patient's skin during an activation cycle; a light source arranged to illuminate the region of the patient's skin from the rear surface of the device; a control system to control the operation of the light source after initiation of the activation cycle; and a power supply to power the light source and the control system, characterised in that the control system is configured, after the initiation of an activation cycle, to vary the output of the light source with respect to time in accordance with a pre-configured output sequence, wherein the output sequence includes an initial stage and a later stage wherein a later stage light dose is output; the initial stage duration being of at least six hours and the output of the light source during the initial stage being set to be zero or below the later stage light dose to avoid the light-induced rupture of endocytic vesicles, to allow time for the antigen and photosensitising agent to reach the target cells; and wherein during the later stage the output of the light source is set to deliver the later stage light dose to activate light-induced rupture of the endocytic vesicles for effecting the delivery of the administered antigen to the cytosol of the target cells, wherein the later stage light dose is at least 0.05 J/cm 2 and is delivered over 5 minutes to 15 hours at an intensity of 0.5 mW/cm 2 or less.