IDO1 and head and neck squamous cell carcinoma: IDO1 inhibitor induced immune cell migration towards cancer cells in HSC‐3 and two HNSCC patient samples, which could change the tumour from “cold” to “hot” and enhance the efficacy of other immunotherapeutic drugs in combination. This in vitro 3D microfluidic chip assay could be used to further test immunotherapeutic drugs against patient samples