Breast cancer immune response. 3D transparent tumour tomography image of a breast cancer specimen, showing T cells (yellow, red and blue) attacking the tumour after treatment. This is a HER2-positive breast cancer (human epidermal growth factor receptor 2). The cytotoxic T cells are defined by their cluster of differentiation protein co-receptors: CD3 (yellow), CD8 (red) and CD31 (blue). The tumour had been treated with radiation and a PD-L1 (programmed death-ligand 1) immune checkpoint blockade therapy. Studying how such anti-tumour immune responses can be triggered may lead to better treatments. This image is of a mouse model of breast cancer.