This image was taken to study the effect of ultrasound on the skin immune cells (labeled green) and dispersion/penetration of vaccine antigen (labeled red) into the skin and draining lymph node, along with investigating the uptake of antigen by immune cells (red and green co-localized cells). Low frequency ultrasound (at 20kHz) applied through an aqueous medium on the skin results in bubble formation which collapse to generate microjets impinging on the skin. Microjet impingement on skin activates the skin immune cells and also enhances penetration of topically applied vaccine into skin. This particular image shows the skin draining lymph node.