During my research training, I became intrigued by the vast impact that our immune system has on our body. On one hand, when out of control our immune system is involved in nearly every disease, however most fascinatingly it is possible to educate our immune system to either reverse, stabilise and even protect us from disease. Its power was already shown in the 18th century when vaccinated people where protected from small pox, and it is still extremely relevant today where vaccins are given to reverse the COVID-19 pandemic.Throughout my research career, I have investigated the function of many different types of immune cells, including dendritic cells (DCs), T-cells, and macrophages. I developed a great interest in how ‘good’ cells can turn ‘bad’ and how we can reverse this.Our the last couple of years, together with my group, we focused, amongst others, on the role of Th17-cell induction by DCs and Th17-cell plasticity in the orphan diseases sarcoidosis. We found that Th17.1-cells appear predictive to develop chronic sarcoidosis. These Th17.1-cells could develop due to intrinsic aberrations of Th17-cells, like low CTLA-4 expression, or to extrinsic factors, such as high production of Th17.1-promoting cytokines by DCs and macrophages. These findings are relevant to other diseases where Th17-cells play an important role. And might also learns us how to educate cells in our immune system not to turn 'bad'.