How coal tar repairs the skin barrier

Many people with eczema make too little filaggrin, a protein the skin needs to build a working barrier. This study grew three-dimensional skin models from the cells of eczema patients and applied coal tar to them. Coal tar activated a receptor called AHR, which drove skin cells to mature properly and restored filaggrin production in cells genetically short of it back to normal levels. When the researchers silenced the AHR receptor, the effect disappeared completely — which is what turns this from an association into a demonstrated mechanism. Coal tar also reduced the fluid build-up and cell death that inflammatory signals normally cause in these models. This is a satisfying piece of science: it gives a molecular explanation for a remedy used since antiquity. But it measures proteins in laboratory tissue, not how much better anyone's eczema got, and it has never been followed by a modern treatment trial.